Corrected_ATPL_Meteorology[1].pdf

ATP Meteorology FLIGHT TRAINING COLLEGE ATP DOC 9 Revision : 1/1/2001 Version 5

ATP METEOROLOGY

ATP Meteorology FLIGHT TRAINING COLLEGE ATP DOC 9 Revision : 1/1/2001 Version 5

INDEX

ATP METEOROLOGY

1. The Atmosphere 01 2. Stability 17 3. Wind 27 4. Airmasses 47 5. Clouds 49 6. Visibility 59 7. Precipitation 69 8. Fronts and Pressure Systems 73 9. Thunderstorms 109 10. Turbulence 119 11. Ice 127 12. Climatology 137 13. Weather Forecasts 149

Annex A Sample Exams 209 Annex B Answers to Questions 239

Copyright © 2001, Flight Training College of Africa All Rights Reserved. No part of this manual may be reproduced in any manner whatsoever including electronic, photographic, photocopying, facsimile, or stored in a retrieval system,

without the prior permission of Flight Training College of Africa.

ATP MET FLIGHT TRAINING COLLEGE ATP DOC 9 Revision : 1/1/2001 Version 5

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CHAPTER 1

THE ATMOSPHERE The Earth is surrounded by a layer of air, the atmosphere. All weather occurs within the atmosphere, and meteorology is the study of this weather. The atmosphere is made up of a mixture of gases, principally nitrogen (78%) and oxygen (21%). Water is present in the atmosphere, in either vapour, liquid or solid form. Particles of dust, smoke and other impurities are also held in suspension in the air. The atmosphere may be conveniently sub—divided into the layers shown. It is the troposphere which is of special interest, since it contains the vast majority of weather.

Within the troposphere the temperature of the air tends to decrease uniformly with height, until the tropopause is reached. The tropopause is a marked boundary between the troposphere and the stratosphere, and it is at this level that the decrease of temperature with height ceases quite abruptly. Within the lower layer of the stratosphere the temperature remains reasonably constant with increasing height, and in fact the temperature immediately above the tropopause is likely to be a few degrees higher (warmer) at high latitudes than it is at low latitudes.


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Towards the top of the stratosphere the temperature actually tends to rise significantly due primarily to the presence of ozone , which is a very strong absorber of ultraviolet radiation. The height of the tropopause varies with latitude and season. In general it is lowest at the poles and highest at the equator and in latitudes higher than 30° it is lowest in winter and highest in summer. Nearer to the equator the seasonal trend is reversed and the tropopause is slightly higher in January than in July. The table below gives approximate mean values for the height of the tropopause over the North Atlantic and adjacent areas.

January July Pole 28 000 ft 32 000 ft

60N 30 000 ft 35 000 ft 30N 55 000 ft 53 000 ft Equator 57 000 ft 55 000 ft

It is important to remember that the tropopause is the level at which temperature ceases to decrease with height. The lowest tropopause temperatures will therefore be found where the tropopause is highest. Appreciate that, since air is compressible, approximately 75% of the mass of the atmosphere is contained in the troposphere, the air above being very much rarefied by comparison. When studying the atmosphere it is convenient to examine the factors which influence weather before moving on to consider wind, cloud and the weather itself. These important factors are heat, pressure and moisture and of these heat (in the form of temperature differences) is the most influential in determining climate. The Standard Atmosphere It is sometimes convenient to compare the state of the atmosphere as it exists at a given place and time with a standard atmosphere. A standard atmosphere is also necessary as a datum to which pressure instruments such as pressure altimeters may be calibrated. The International Standard Atmosphere Mean sea level temperature +15°c Mean sea level pressure 1013.25 hPa Mean sea level density 1225 grams per cubic metre The temperature is assumed to decrease from mean sea level at the rate of 1.98°c per 1000 feet (6.5°c per 1000 metres) up to an altitude of 36,090 ft (11 km) and thereafter to remain constant at -56.5°c up to 65,600 ft (20 km). Above 65,600 ft the temperature is assumed to rise at a rate of 0.3°c per 1000 ft (1°c per 1000 metres) up to an altitude of 105,000 ft (32 km). The Jet Standard Atmosphere The mean sea level values of temperature, pressure and density are identical to those of the International Standard Atmosphere, however the temperature lapse rate is assumed to be 2cْ per 1000 feet with no tropopause. In other words, the temperature in the Jet Standard Atmosphere at 40,000 ft is -65C (as compared with -56.5C in the International Standard Atmosphere).


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Heat The presence of quantities of heat in the atmosphere has a very marked influence on the weather. Temperature In meteorology temperature is measured in degrees Celsius (°c). For all calculations, however, temperatures in degrees Absolute (°A) are required. The size of a one degree step is exactly the same on the Celsius scale as on the Absolute scale. The difference is the starting point, such that:

0°A = -273°c 273°A = 0°c

The Kelvin scale has the same starting point as the Absolute scale and the value of each unit in the Kelvin scale is in effect 1° Celsius or Absolute. When using the Kelvin scale, however, the degree sign is omitted. Latent Heat Latent heat plays a very important part in meteorology, from airframe icing to hurricanes, and so an understanding of the principle is important. As has already been said, water exists in the atmosphere in three states, solid, liquid and vapour. For a change of state to take place from solid to liquid, liquid to vapour or solid to vapour heat energy (latent heat) must be supplied in order to bring about this change of state. Conversely, when water is changing state in the opposite direction (vapour to liquid, liquid to solid or vapour to solid) an equivalent amount of heat energy (latent heat) is released.

The change of state of water from solid to liquid state is termed melting (or fusion). The change of state from liquid to solid is termed freezing. The change of state of water from liquid to a vapour is termed evaporation and in the

reverse direction condensation. The change of state of water directly from a solid to a vapour is termed sublimation. The change of state of water directly from a vapour to a solid is termed deposition (or

sometimes as sublimation). Latent heat is involved in the change of state of any matter, however it is the latent heat involved in the change of state of water which is of concern to us in our study of meteorology. The unit of heat used is the calorie, which is the amount of heat required to raise the


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temperature of one gram of water (or ice), by 1°C. For water the latent heat of fusion (ice to liquid water or reverse) is 80 calories of heat energy per gram. In order to raise the temperature of 1 gram of ice from -10C to 0C it will therefore be necessary to supply this ice particle with 10 calories of heat. In order to change this ice particle into a water droplet it is now necessary to supply a further 80 calories of heat. During this melting process the temperature of the water will remain at 0C, since the heat which is being supplied is being used to achieve the change of state. Any subsequent supply of heat will cause the temperature of the water droplet to rise. The latent heat of vaporisation (evaporation/condensation) is 540 calories per gram at 100C, rising to 600 calories per gram at 0C. The latent heat of sublimation/deposition (ice direct to vapour or vapour direct to ice is 680 calories per gram. Latent heat will be further considered in the sections on stability, cloud formation and icing. Heating of the Atmosphere The sun is the primary source of heat for the atmosphere. The small amount of heat released from the core of the Earth can be disregarded. Because of the high temperature of the sun the incoming solar radiation, which is termed insolation, has a short wavelength. The ozone layers in the upper atmosphere absorb small amounts of this insolation, primarily the harmful ultra—violet radiation. Further small amounts of this short wave radiation are absorbed by water in the atmosphere, by the carbon dioxide within the atmosphere and by solid particles in suspension in the air. As well as absorbing solar radiation, clouds will also reflect some of this radiation back into space. The amount of energy which is reflected will of course depend on the extent, depth and nature of the cloud.

The solar energy which reaches the surface of the Earth is now either absorbed or reflected. The proportion of energy which is absorbed rather than reflected will depend on the nature and specific heat of the surface.


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Only a small amount of heat is absorbed into the atmosphere directly from incoming insolation. The atmosphere must therefore be heated indirectly by the sun. That is to say that the sun heats the surface of the Earth and that the heat from the surface is then transferred to the atmosphere by means of conduction (the air in contact with the surface is warmed) and convection (hot air at the surface rises into the atmosphere). Further atmospheric warming occurs when warm air at the surface rises, cooling as it does so, until the water vapour within the air condenses out as either water droplets or ice crystals, thus releasing latent heat. Finally, relatively long wave radiation from the Earth’s surface (long wave because the Earth’s surface is very much cooler than the surface of the sun) adds to the heating of the atmosphere. Because this terrestrial re-radiation is long wave, it is rather more readily absorbed by the water and the carbon dioxide contained within the atmosphere. Temperature Variation The temperature of the air close to the Earth’s surface will vary because of many factors. The mean surface air temperature of the poles is much lower than at the equator. A further fact contributing to low temperatures at high latitudes is that snow and ice reflect a high proportion of the insolation, and that much of the heat which is absorbed is used as latent heat to melt the snow or ice without an increase in temperature.

If you appreciate that the sun appears to circle the Earth at northerly latitudes between April and September and at southerly latitudes between October and March, then you should also understand why seasonal variations of mean surface air temperature occur. The temperature of the surface will tend to change diurnally, that is to say over a 24 hour period. Below shows an idealised diurnal curve of temperature variation, for a dry land surface, with cloudless skies and no wind.

Shortly after sunrise, the amount of insolation begins to exceed the amount of terrestrial re-radiation, and the temperature rises. By 1400 local mean time the sun has passed through the zenith and the angle of incidence of the incoming rays decreases. The amount of terrestrial re-radiation now exceeds the amount of insolation and the temperature falls.


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Over the sea rather more of the incoming solar radiation is reflected, and much of the heat which is absorbed does not result in a rise in temperature, since it is required as the necessary latent heat to facilitate the process of evaporation of the sea surface into the atmosphere. Additionally, the insolation will penetrate to a depth of several metres, causing a minimal overall rise in temperature. As the sea re-radiates long wave heat energy, the plentiful supply of water vapour near the surface readily absorbs this energy. The heat loss from the surface layer is therefore small, and the result in a diurnal variation in surface air temperature over the sea seldom exceeds 1C. Over the land the diurnal variation of temperature is reduced by cloud cover. Substantial layers of stratus cloud will reduce the amounts of insolation reaching the Earth’s surface, and will quite effectively blanket the Earth at night. This blanket of cloud absorbs much of the long wave terrestrial radiation, and re-radiates it back to the surface. High surface winds will also flatten the diurnal curve, since the wind causes the air at the surface, which is changing temperature diurnally, to mix with the air above, which is likely to be at a far more constant temperature since it is neither heated or cooled (directly) by varying surface temperatures. From a consideration of these factors it should be apparent that the greatest diurnal variation of temperature will occur inland (rather than on the coast) when skies are clear. In addition, this variation is, on average, greater in summer than in winter because the higher daytime temperatures during the summer result in greater heat loss at night (the hotter a body the greater the rate at which it gives up heat). Surface Air Temperature Surface air temperature is measured at a point 1 .25 metres above the ground, where the free flow of air is unrestricted. The thermometer is protected from the direct rays of the sun, the box used to protect the thermometer from the elements is called a Stevenson screen. Temperature Units Temperature is measured either in Celsius , Fahrenheit, or Kelvin. It must be known that : 0°c = 273°K Celsius to Fahrenheit = 5/9 (F-32) Fahrenheit to Celsius = -32 ÷ 9 5


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Environmental Lapse Rates If the ambient air temperature (the actual air temperature) is measured at various heights in the atmosphere, and a graph is plotted showing temperature against height, then a graph of environmental temperature lapse rate (ELR) is produced. Below you can see the three distinct types of ELR.

Figure (a) shows a normal ELR, normal since we know that on average the temperature of the air decreases with height at around 2C per 1000 feet. Figure (b) shows a situation where the temperature remains constant through a given layer of air, this is known as an isothermal layer. (c) shows a situation where the temperature is increasing with height, and this is known as an inversion. Temperature Deviation It is often convenient to express the actual (ambient) temperature at a point in the atmosphere by comparing it with the temperature which would exist at the same point in either the International Standard Atmosphere (ISA) or the Jet Standard Atmosphere (JSA). For convenience it is sufficiently accurate to use a temperature lapse rate of 2°C/1 000 ft (rather than 1 .98°C/1 000 ft) when working with the ISA. Example: An aircraft is flying at Flight Level 250 (25,000 ft). The ambient temperature is –37°C. Determine the temperature deviation from ISA. Solution First calculate the temperature which would exist at 25,000 ft in the ISA (2C/1000’). At 25,000 ft in the ISA the temperature would be (25 x 2) 50C colder than at MSL. The temperature in the ISA at MSL is always +15C in either of the standard atmospheres. Therefore the temperature (ISA) at 25,000 ft would be (+15C - 50C) = -35C. Temperature deviation is a statement of the deviation of the ambient condition from the standard condition.


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Therefore: ISA temperature (25,000 ft) = -35C Ambient temperature (25,000 ft) = -37C Temperature deviation = ISA -2C Example 2 The ambient temperature at 17,500 ft is -16C. Express this temperature as a deviation from ISA. Solution ISA temperature (1 7,500 ft) = +15C - (17.5 x 2C)

= +15C-35C Ambient temperature (17,500 ft) = -16C Temperature deviation = ISA +4C Example 3 At FL 250 the temperature is given as ISA -9C. Determine the ambient temperature. Solution

ISA temperature (25,000 ft) = +15°c - (25 x 2C) = +15C — 50C = -35C

ISA temperature deviation = -9C (The ambient temperature is 9C colder than the standard temperature)

Ambient temperature = -35C- 9C = -44C

When working temperature deviation problems above 36,000 ft it is essential to know whether you are using ISA or JSA, since one has a tropopause and the other doesn’t. To illustrate, an ambient air temperature of -60C at 40,000 ft expressed as a deviation from standard is ISA -3.5C and JSA +50. Example 4 At FL 430 the temperature is given as ISA +3C. Express this temperature as a deviation from the Jet Standard Atmosphere. ISA temperature (43,000 ft) = -56.5C Deviation from ISA = +3C Ambient temperature (43,000 ft) = -53.5C JSA temperature (43,000 ft) = +15C - (43 x 2C)

= +15C – 86C = -71C

JSA temperature deviation = +17.5C


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Pressure Barometric pressure is the force exerted by the atmosphere. Pressure acts in all directions, upwards and sideways as well as downwards. It is convenient to imagine pressure as the weight of the column of air above a given point. The point in question may be the surface of the Earth, or the altimeter capsule within an aircraft.

The units of pressure are force divided by area, for example pounds per square inch, or dynes per square centimetre. The unit of pressure used by South Africa is the hectopascals (hPa), which is the pressure exerted by 1000 dynes per square centimetre. In the continuing process of international standardisation (some would say confusion), there is a gradual move towards expressing atmospheric pressure in Hectopascals (hPa or HPA). The good news is that there is no difference between the millibar and the hectopascal scales. Only the name of the unit of measurement has been changed.

Pressure Datums The subscale of an altimeter is normally set to one of three datums: QFE The pressure observed at the airfield datum point. QNH The QFE reduced to mean sea level (MSL) pressure using the standard

atmosphere temperature lapse rate. The pressure altimeter is calibrated to the standard atmosphere, and so when QNH is set on the altimeter subscale the instrument indicates the airfield elevation. We talk above of reducing QFE to QNH. It is however a reduction in height which results in an increase in pressure when changing OFE to QNH for an airfield which is above MSL.

1013 hPa When flying above the transition altitude it is normal to set 1013 hPa on the

altimeter subscale and maintain a flight level. When 1013 is set on the subscale, the height shown on the altimeter when the aircraft is on the ground is known as the QNE value.


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Meteorologists determine a mean sea level pressure which is more accurate than the QNH by reducing the QFE to mean sea level using ambient rather than standard atmosphere temperature lapse rates. The MSL pressure thus obtained is termed the QFF. Isobars Isobars are lines joining points of equal barometric pressure. Surface isobars are the most commonly used of all constant pressure lines, but the name is misleading. Surface isobars, by definition, join points of equal mean sea level pressure rather than points of equal surface pressure. In other words, surface isobars join points of equal QFF rather than points of equal QFE.

The surface chart above shows surface isobars (at 2 hPa intervals which is normal) forming the characteristic patterns outlined below

Anticyclones or highs (H) Depressions or lows (L) Ridges (R), which are elongated highs. Troughs (T), which are elongated lows. Cols (C), which are areas surrounded by two diagonally opposed lows and two diagonally

opposed highs.


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Pressure Gradient Pressure gradient is a measure of the rate of change in pressure with distance. It is normally expressed as the change in pressure in hPa’s per 100 nm measured at right angles to the isobars. The windspeed is related to the pressure gradient, the closer the isobars the stronger the windspeed, but this covered fully in subsequent chapters. Isallobars Stations measuring and reporting barometric pressure do so at fixed intervals. It is therefore a simple matter to determine the rate of change of pressure per hour at each reporting station. Lines joining points of equal rate of change of pressure are known as isallobars. Isallobars help the forecaster to predict the future path of pressure systems. Density Barometric pressure may be thought of as the weight of air above a point, it is clear therefore that pressure must reduce as height is increased. The rate of change of pressure with height depends upon the density of the air concerned. Density is defined as mass per unit volume, for example grams per cubic metre. As air is heated it expands, and the same mass of air then occupies a larger volume. In other words the density of the air has reduced. Conversely, when air cools it contracts, now the same mass of air occupies a smaller volume and the density of the air has increased. Consider two columns of air, one warm and one cold. The cold column will be denser than the warm column and therefore the rate of change of pressure in the cold column would be greater than in the warm column. Assuming that the surface pressure is the same at both columns, then any given pressure will occur at a lower level in the cold column than in the warm column. Of course a pressure altimeter senses a pressure and indicates a level which it is calibrated to associate with that pressure. Consequently, since pressure altimeters are calibrated to the ISA, they will tend to overread in air which is colder than standard (the dangerous case), and underread in the air which is warmer than standard (the safe case).


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Moisture and the Atmosphere Water exists in the atmosphere in one of three states, as water vapour, as liquid water droplets, or in its solid state as ice crystals or occasionally hail stones. As already discussed, the change of state of water involves the absorption or release of latent heat, and this is a very important aspect of meteorology. Water vapour is present in dry air. Here the term dry implies that the air is unsaturated rather than totally without moisture. Water vapour is absorbed by the air by the process of evaporation. Whenever dry (unsaturated) air flows over a moisture source, some of the loosely bound molecules of water at the surface are absorbed into the air as vapour. The dryer and hotter the air, the greater the rate of evaporation. The ability of the air to hold this water in vapour state depends on the temperature of the air. The warmer the air, the greater the amount of water vapour that it can hold. As air with a given moisture content is cooled its ability to hold vapour diminishes. The critical temperature is the one at which the amount of vapour which the air is capable of holding becomes equal to the amount of vapour which the air is actually holding. At this point the air has just become saturated, and the critical temperature is called the dew-point. The dew-point is therefore defined as that temperature to which moist air must be cooled in order to become saturated. Any further cooling below the dew-point will produce condensation, in the form of fog or cloud. Relative Humidity The actual water vapour content of the atmosphere is known as the absolute humidity. A more useful expression of the state of the atmosphere in terms of the moisture content is relative humidity. Relative humidity (RH) is a statement of the degree of saturation expressed as a percentage and may be defined as:

The ratio of the actual water vapour content (WVC) of the air at a given temperature to the maximum amount of water vapour which the air could hold at that temperature, expressed as a

percentage. To give you some idea of the amount of water vapour in question, the weight of water vapour required to produce saturation at a temperature of +30C (at MSL) is approximately 27 grams per kilogram of air. At a temperature of +15C (at MSL) the weight of vapour required for saturation is reduced to approximately 11 grams per kilogram of air, whilst at 0C (at MSL) the critical figure is only 4 grams per kilogram. From the preceding paragraph it can be seen that air at low level having a temperature of +15C and a water vapour content (WVC) of 4 gms/kg would have a relative humidity of approximately 36% since: RH (%) = actual WVC x 100 maximum WVC for the given temperature = 4gm/kg x l00

11 gm/kg = 36%


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Were this air to be cooled to zero degrees the relative humidity would be 100% and the air would be saturated. It is important to appreciate that cooling air below the dew—point must result in condensation since (ignoring the condition of super—saturation which is very rare) the relative humidity of the air cannot exceed 100%. Consequently the excess water vapour (on the top line of the equation) must change state. The Wet and Dry Bulb Thermometer A wet and dry bulb system is used to determine surface air temperature, relative humidity and dew—point. Note however that the wet bulb temperature is not the dew—point temperature, except when the air is saturated. The dry bulb thermometer measures the temperature of the free air. A wet bulb thermometer is a normal thermometer, the bulb of which is wrapped with a single layer of muslin, kept continually moist by means of distilled water which is supplied from a reservoir through a short wick. Any evaporation is shown by a lower wet bulb temperature, due to the extraction of the latent heat of evaporation from the bulb. The drier the air the greater the rate of evaporation, and the larger the amount of heat removed from the bulb. A large difference between dry and wet bulb temperatures therefore indicates dry air, or low relative humidity. Alternatively, identical wet and dry bulb temperatures indicate that no evaporation is occurring, that the air is saturated, and that therefore the relative humidity is 100%. The wet bulb temperature may be defined as the lowest temperature to which air may be cooled by the evaporation of water. The wet and dry bulb thermometers are collectively termed a hygrometer. DEWPOINT The dewpoint is the temperature to which a volume of air must be cooled at a constant pressure for saturation to occur. This is not to be confused with wet bulb temperature…


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Revision Questions 1. An aircraft is at FL220 with the altimeter set. The pilot omits to reset the altimeter for

landing. The destination airfield has an elevation of 580m, with a QNH of 1026. 1 hPa. After landing, the altimeter reads:

a) 2 285’ b) 1 517’ c) 1 900’

2. During an altimeter serviceability check, the following indications were observed. Airfield

elevation 5327’, apron elevation 5 306’. The altimeter with the QFE set reads 80’. The instrument error is:

a) 101’ b) 59’ c) 80’

3. The amount of water vapour in a mass of air expressed as a percentage of the total amount

of water vapour that the mass of air could contain if it was saturated at the same temperature and pressure, is known as the:

a) absolute humidity; b) relative humidity: c) specific humidity.

4. Rising air becomes colder because the

a) pressure decreases with height and the air expands; b) surrounding air is colder at higher levels; c) water vapour in the air becomes less.

5. You are flying at an indicated height of 2000 feet from a high pressure to a low pressure

system. It you maintain the indicated height your true height will:

a) increase b) stay the same c) decrease.

6. The earth’s weather changes are primarily due to one of the following:

a) variation of solar energy received at the surface of the earth; b) movement of the airmass; c) pressure variations over the earth’s surface.

7. The amount of water vapour which may be held in suspension in the atmosphere depends

largely on the following:

a) The air temperature. b) The stability of the air. c) The dew point temperature.


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8. The temperature at an airfield at 3 000’ AMSL is 84° F. This represents:

a) ISA +20 C b) JSA+15C c) JSA+17C

9. Select the correct statement:

a) 32°C=0°F b) C=5/9(F-32) c) F=5/9( C+32 )

10. A Stevenson screen is used in the measurement of:

a) Wind velocity. b) Temperature. c) Rainfall.

11 . Landing at Johannesburg International, (elevation 5500’), you are given a QFE

of 840 hPa. The QNH is:

a) 1023hPa b) 1017 hPa. c) 1003 hPa.

12. An increase in relative humidity with no change in temperature or pressure

will cause:

a) No change in air density. b) A slight decrease in density altitude. c) A slight increase in density altitude.

13. An aircraft flying at a constant flight level from an area of high temperature to

an area of low temperature will experience:

a) An increase in density altitude. b) A decrease in density altitude. c) An increase in density altitude unless there is a substantial increase in

relative humidity. 14. At 11:00 Z the pressure altitude at an airfield is 1 250’ and the OAT is 32°C.

At 22:00 Z the pressure altitude is 1750’ and OAT 15°C for the same airfield. As a result. the density altitude will:

a) Increase by 300’. b) Decrease by 1309’. c) Decrease by 750’.

15. Density altitude is equal to:

a) True altitude corrected for temperature. b) Indicated altitude when the altimeter sub scale is set to the QNH. c ) Pressure altitude corrected for temperature.


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16. An aircraft is flying at FL240. Airfield elevation is 1560 m. the QNH is

1 021 hPa. The height of the aircraft AGL is:

a) 18883’ b) 22673’ c) 19116’

17. Airfield elevation is 1 250m; the QNH is 1024 hPa. OAT is 34°C.

The density altitude is:

a) 6 946’ b) 7 268’ c) 7 590’

18. The lowest temperature to which air can be cooled by evaporation into it is

a) Dewpoint. b) Wet bulb temperature. c) S.A.L.R.


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CHAPTER 2

STABILITY The stability, or otherwise, of the air is one of the primary factors which determine the type of weather experienced. A stable atmosphere will, depending on the temperature and humidity, give either fine weather with clear skies, or widespread stratiform cloud with drizzle, or perhaps fog. Alternatively an unstable atmosphere will give cumuliform cloud and showers, possibly heavy with thunderstorms. In order to assess the state of stability or otherwise of the atmosphere it is necessary to compare the environmental lapse rate (discussed in Chapter 1) with the appropriate adiabatic lapse rate. The meteorologist is able to make such comparisons on a special graph of temperature, humidity and pressure values called a tephigram. The diagrams which follow are simplified versions of this graph. Adiabatic Lapse Rate If a parcel of air moves vertically within the atmosphere the pressure exerted on the parcel by the column of air above it will necessarily change, the pressure decreasing as the parcel rises and increasing as the parcel descends. As a consequence of the changing pressure the temperature of the air contained within the parcel will also change, the temperature falling as the parcel ascends and rising as the parcel descends. The change of temperature which occurs solely because of change of pressure is known as adiabatic heating or cooling as appropriate. When considering adiabatic lapse rates it is assumed that no heat energy will flow between the parcel of air and the surrounding environment. Dry Adiabatic Lapse Rate The rate at which unsaturated (dry) air will change temperature, solely due to change of pressure, when moved vertically within the atmosphere, is 3°c per 1000 ft of vertical displacement. On the next page you see a graph of altitude (vertical scale) against temperature (horizontal scale). The height range shown is from mean sea level to 15,000 ft and the temperature range is from –30°C to +30°C. It is necessary to become familiar with the graph and its uses in order to understand much of which follows. Locate the straight lines running diagonally from bottom right to top left of the graph, the bottom left side being labeled DALR for dry adiabatic lapse rate. Check that the lapse rate shown is in fact 3°c/1 000 ft. For example the DALR line which starts at +30°c at MSL shows 15°C a: 15,000 ft. A 15,000 ft change of height at 3°C/1 000 ft is a 45°c change in temperature, which checks.


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Saturated Adiabatic Lapse Rate The rate at which saturated air changes temperature, when forced to move vertically within the atmosphere is known, logically, as the saturated adiabatic lapse rate (SALR). The SALR is quantified as being approximately 1.5°C/1000 ft in the lower atmosphere in temperate latitudes. The reason for the difference between DALR and SALR is latent heat. The parcel of air here being considered is saturated, that is to say that it is incapable of holding any more water vapour at its present temperature. Furthermore the ability of the parcel of air to hold water vapour will diminish as its temperature decreases. If the parcel of air is forced to rise its temperature will drop, condensation will occur, latent heat will be released and will diminish the rate at which the temperature drops with increase of altitude. At the low temperatures and pressures of high altitude the water vapour content which air is capable of holding is very small. Under these circumstances the amount of water vapour released to condensation is very small, the amount of latent heat released is negligible, and the SALR comes close to the DALR value of 3°c/1 000 ft. A similar comparison can be drawn between the warm air of low latitudes and the cold air of high latitudes. Air which is warm is capable of holding relatively large amounts of water in vapour form. Once this parcel of ascending warm air has cooled beyond its dewpoint, relatively large amounts of latent heat will be released as the plentiful supply of water vapour condenses out. The SALR of this parcel of warm air will therefore be low (perhaps 1.2°C/1000 ft near the surface), compared with its colder cousin at high latitudes (perhaps 1.8°C/1000 ft near the surface).


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Return now to the previous diagram and locate the curved saturated adiabatic lapse rate (SALR) lines on the graph. Satisfy yourself that a tangent to the curves close to mean sea level gives a rate of approximately 1.5C/1 000 ft, but at 15,000 ft the rate is closer to 2.5C/1 000 ft. due to the decreasing amount of latent heat released by condensation. Stable Air Stable air, by definition, is an air mass within which a parcel of air may be displaced vertically, and will tend to return to its original level. An example of dry air in a stable state is shown below.

In this case the air is dry and therefore the parcel, which is lifted through 5000 ft, will cool at the DALR. The parcel will therefore cool from +15C to 0C in this example. The environmental lapse rate, the actual temperature of the air surrounding the parcel, is shown to the right of the DALR line. At 5000 ft the ELR line shows the temperature of the free air to be +5°C. The lifted parcel is colder, and therefore denser, than the surrounding air. The parcel will consequently sink back to its original level when the lifting force is removed. The air is therefore stable.


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This time the lifted parcel of air is saturated throughout the ascent and has therefore cooled at the SALR of 1.5C/1 000 ft in the lower atmosphere from +10C at MSL to +2.5C at 5000 ft. Again the ELR lies to the right of the relevant adiabatic line. The parcel of air is colder and therefore denser than its environment, and the parcel will sink once the lifting force is removed.


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The previous graph shows an ELR lying to the right of both the DALR and the SALR. In this situation the atmosphere is stable, regardless of whether the lifted parcel of air is dry or saturated. In either event the parcel will be colder and therefore denser than its environment at the top of the lifting layer, and will therefore subside. Unstable Air The next graph shows dry air in an unstable state. Now the ELR lies to the left of the relevant adiabatic lapse rate line, which in this case is the DALR since the air is dry and remains so throughout its ascent within the lifting layer. At the upper limit of the lifting layer (in this case 5000 ft) the parcel of air is warmer and therefore less dense than the environment. The parcel will therefore continue to rise, seeking air with an actually low density.

The next diagram shows saturated air in an unstable state. Now the ELR lies to the left of the SALR and consequently a parcel of air which remains saturated throughout is assisted ascent will be less dense than its environment on reaching the upper limit of the lifting layer. Again, therefore, the parcel will continue to rise once the lifting force is removed.


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Absolute instability exists when the ELR lies to the left of both the DALR and the SALR. In this event a parcel of air will be unstable and will continue to rise beyond the lifting layer regardless of whether it be dry, saturated, or start dry and become saturated during its ascent within the lifting layer. Absolute instability is shown below.

To complete the picture consider the situation where the ELR lies between the DALR


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and the SALR, as shown next. In this the air is said to be conditionally unstable. This is because the air is only stable as long as it is dry. Very shortly after saturation occurs during the lifting process the lifted parcel will become unstable.

Neutral stability exists when the parcel of air exists at the same density as the environmental air surrounding it and therefore moves neither up or down.

SUMMARY

If ELR < SALR the atmosphere is always stable

If SALR < ELR < DALR the atmosphere is stable for unsaturated air , but unstable for saturated air

If ELR > DALR the atmosphere is always unstable

**In cases where the ELR equals the SALR or DALR, the possibility of neutral stability should be investigated.


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QUESTIONS 1. The characteristics of UNSTABLE air are:

a) turbulence and good surface visibility; b) turbulence and poor surface visibility; c) smooth conditions and good surface visibility.

2. The saturated adiabatic lapse rate is less than the dry adiabatic lapse rate

because:

a) its rate of ascent is less; b) latent heat is released during the saturated adiabatic process; c) water vapour does not cool as rapidly as air.

3. Absolute instability exists in the atmosphere when:

a) the ELR is greater than the DALR; b) the ELR is less than the SALR c) the ELR lies between the DALR and the SALR.

4. The SALR<ELR<DALR. This conditions is known as:

a) Absolute instability; b) Conditional instability; c) Absolute stability.

5. Conditional instability exists in the atmosphere when

a) the ELR is greater than the DALR: b) the ELR is less than the SALR; c) the ELR lies between the DALR and the SALR.

6. The characteristics of unstable air are: Visibility Type of precipitation Type of clouds a) poor drizzle stratus b) good showers cumulus c) good steady rain nimbostratus 7. A cold air mass moving over a warm land mass will:

a) become stable; b) become unstable; c) will cause poor surface visibility.

8. Which are the characteristics of a cold air mass moving over a warm surface:

a) Cumuli form clouds, turbulence and good visibility. b) Stratiform cloud, smooth air and poor visibility. c) Cumuli form cloud, turbulence and poor visibility.


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9. The characteristics of stable air are:

a) Poor visibility and steady precipitation from stratus type cloud. b) Poor visibility and intermittent rain from cumuli form type cloud. c) Good visibility and steady precipitation from cumuli form type cloud.

10. The type of cloud expected to form when unstable moist air is forced to rise

over a mountain is most likely to be:

a) Stratified clouds with intermittent showers. b) Layer type cloud with little vertical development. c) Vertical development type cloud.

11. What type of cloud is most likely to form under conditions of stable moist air:

a) Fair weather cumulus cloud. b) Stratiform cloud. c) Cumuli form type cloud.

12. Moist stable air flowing upslope may be expected to produce:

a) stratus type cloud; b) thunderstorm and showers; c) a temperature inversion.

13. How can stability of the atmosphere be determined:

a) From the surface temperature. b) From the environmental lapse rate. c) From the dry’ adiabatic lapse rate.

14. The structure and formation of different cloud types which form as a result of

air that is forced to rise depends upon:

a) the stability of the air before it is forced to rise; b) the method by which the air is forced to rise; c) the amount of humidity present after lifiing occurs.

15. What are some of the characteristics of unstable air:

a) Turbulence and good surface visibility. b) Nimbostratus cloud and poor surface visibility. c) Nimbostratus cloud and good surface visibility.

16. Saturated air that is forced to rise will cool at an average rate of:

a) 1.98C /1 000 ft; b) 3C/1000ft; c) 1.5C/1000ft.


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17. If the SALR is 1.7°C/1 000 and the ELR is 1.625°C/1 000’, then the

air mass is likely to be:

a) Conditionally unstable. b) Absolutely stable. c) Absolutely unstable.

18. The dry bulb temperature on the surface is 8 C. At 4 000’ AGL the

temperature is 4 C. The air is:

a) Stable. b) Unstable. c) Conditionally unstable.


a) An inversion is a decrease in temperature with a decrease in altitude. b) During the adiabatic process there is no change in pressure. c) The temperature remains constant during the adiabatic process.

20. ELR> DALR. This would indicate:

a) Absolute stability. b) Absolute instability. c) Conditional instability.


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CHAPTER 3

WIND Wind is the horizontal motion of air. Wind velocity quantifies this horizontal motion in terms of the direction from which the wind is blowing, and the speed at which it is blowing. Buys Ballots law states that if an observer stands with his back to the wind low pressure is on his/her right in the southern hemisphere, and on his/her left the northern hemisphere. Wind which changes direction, either at one given point with the passage of time, or at one point when compared with another, is said to veer or back, depending on the direction of the change. When the wind direction changes in a clockwise sense (from 090 to 180°) it is said to veer. Conversely when the wind direction changes in an anticlockwise sense (from 330° to 27°) it is said to back. Gusts of wind are increases in the prevailing windspeed of relatively short duration, measured in seconds rather than minutes. They are generally confined to the air near the surface and result, for example, from airflow around buildings. Similarly, a lull is said to occur when the windspeed decreases for a few seconds. The gust factor is often used to warn a pilot to expect turbulence associated with gusting surface winds: The gust factor= The range of fluctuations in gusts & lulls x 100

The mean windspeed If surface observations showed gusts of 35 kt and lulls of 15 kt the mean windspeed would be 25 kt. The range of fluctuations would be 20 kt and so, in this case, the gust factor would be 80%. Squalls are also increases in windspeed, but now the increased windspeed is likely to last for minutes rather than seconds. Squalls are normally associated with the passage of large cumulonimbus clouds, especially when these contain active thunderstorm cells. A gale force wind is said to exist, and a gale warning issued by the Meteorological Office, whenever the windspeed measured 10 metres above the surface has a mean value of 34 kt or greater, or is gusting to 43 kt or more.


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Geostrophic Wind A geostrophic wind is the wind which blows at 2000 feet when the surface isobars are straight and

parallel. Geostrophic winds obey Buys Ballots law and must therefore blow parallel to the isobars as shown . The geostrophic wind blows along the isobars, rather than across them from high to low pressure, because of the presence of the geostrophic force. The speed of the wind is governed by the pressure gradient, which is indicated by the distance between the isobars, the closer the isobars the steeper the pressure gradient and therefore the stronger the wind. All unsteered bodies in motion over the surface of the Earth are subject to a deflecting force, to the right in the northern hemisphere and to the left in the southern hemisphere. This force is generally known, when the unsteered body is moving air, as the coriolis force.


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Below shows how the geostrophic wind is established. A body of air will move initially in the direction H-L under the influence of the pressure gradient (PG), which always acts at right angles to the isobars. As soon as the body begins to move it becomes the veritable unsteered body, and is subject to a deflecting geostrophic force (GF), which acts at right angles to the direction of movement. The body of air therefore follows the curved path and, but for the continuing presence of the geostrophic force, the particle would continue in a straight line. The geostrophic force continues to deflect the air, however, along a curved path(real wind). This process is continued until the air is moving under the influence of a balance between the pressure gradient and the geostrophic force.

Since the pressure gradient and the geostrophic force are balanced, once the geostrophic wind is established, they are necessarily equal in magnitude and opposite in direction. The geostrophic wind therefore blows parallel to the isobars with low pressure to the left in the northern hemisphere and to the right in the southern hemisphere. As latitude decreases the speed of the geostrophic wind increases for a given pressure gradient. The fact that geostrophic wind speed (for a given pressure gradient) increases in magnitude as latitude is decreased would suggest that, at the equator, the smallest pressure gradient would give an infinitely strong geostrophic wind. In fact, the wind does not obey Buys Ballot’s law in low latitudes, and the geostrophic formula is considered to break down within 15° of the equator. The result is that, at low latitudes, the air moves across the isobars from high pressure to low pressure.


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Gradient Wind The geostrophic wind blows at 2000 ft when the isobars are straight and parallel, however more often than not isobars are not straight but curved. Curved isobars surrounding both low and high pressure systems in the northern and southern hemisphere are shown below. In each case the wind at 2000 ft will still obey Buys Ballot’s law, and will therefore curve around the system to remain parallel to the isobars. A wind which follows a curved path in this manner is termed a gradient wind. The fact that the air follows a curved path indicates the presence of a third force in addition to the pressure gradient and the geostrophic force. The third element is the centripetal force, which acts inwards towards the centre of the system, and which constrains the air to follow a curved path.

The previous diagrams graphically shows the three elements of the gradient wind around a low pressure system. The centripetal force is provided by the pressure gradient force (the air could not follow a curved path unless it was moving, and the movement is itself caused by the pressure gradient). Consequently the pressure gradient (in terms of its ability to generate wind speed) is effectively reduced and this results in a reduction in the speed of the gradient wind. This means that when isobars are curved cyclonically around a low pressure area the gradient wind is less than the theoretical (or geostrophic) value for a given pressure gradient. To put it another way, given a constant air density, latitude and isobaric spacing, the wind speed at 2000 ft will be lower when the isobars are curved concentrically around a low pressure system than it would be were the same isobars to be straight and parallel. It should be noted that the above explanation, whilst something of an oversimplification, is an accepted method of describing the gradient wind/geostrophic wind relationship for the purpose of the CAA syllabus.


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Many people find it difficult to accept that, at a given latitude and with the same pressure gradient, the wind around a high will be stronger than the wind around a low, however the statement is correct for the same pressure gradient. Experience tells us that gale force winds occur around depressions whilst the winds associated with anticyclones are usually light, however the reason for this is that steep pressure gradients are found close to depressions (the isobars are tightly packed) whereas slack pressure gradients normally exist in high pressure regions. Wind at the Surface We have now established that in a status quo situation the air at about 2000 ft will blow parallel to the surface isobars. Furthermore we know that, if the pressure is changing, the isallobaric effect will cause the air to tend to flow from high to low pressure. It is now necessary to consider the effect of surface friction on the movement of air moving adjacent to the surface. Below illustrates the relationship between a 2000 ft geostrophic wind and resulting surface wind in the southern hemisphere. The wind at 2000 ft is shown as blowing parallel to the isobars since the pressure gradient force is equal in magnitude but opposite in direction to the geostrophic force.

Taking this horizontal movement of air to the surface will cause the windspeed to decrease because of surface friction. If the windspeed decreases so must the geostrophic force. At the surface the pressure gradient is therefore greater than the geostrophic force and the wind tends to blow in the direction of the stronger force, which is the pressure gradient. The wind at the surface will therefore decrease in speed and back in the northern hemisphere, but decrease in speed and veer in the southern hemisphere, when compared with the wind at 2000 ft. The amount by which the surface wind will decrease in speed, and consequently change direction, depends initially on the nature of the surface. The sea exerts only a small frictional retardation on the air above, and therefore the speed of the wind at the surface is not changed dramatically (perhaps a 25% reduction), and consequently the direction of the surface wind changes only a little (perhaps only 15° removed from the 2000 ft wind, backed in the northern hemisphere, veered in the southern hemisphere). Over the land the frictional retardation is far greater than over the sea. The tendency is therefore for the surface wind to be considerably reduced in speed and changed in direction when compared with the 2000 ft wind above the friction layer. Unfortunately the situation over the land is not quite so clear cut, and it is now necessary to consider the effect of the mixing layer on the surface wind.


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There are basically two factors which will influence the degree to which the air at the surface will mix with the air above. The first is the surface temperature, which will govern the amount of thermal mixing. Over the sea the degree of thermal mixing will remain more or less constant by day and night, because of the practically non-existent diurnal change in surface temperature. Conversely, over the land the degree of thermal mixing is likely to be far greater a noon than at midnight, especially with clear skies. This means that during the day the air at the surface will mix with air above through a fairly deep layer. This reduces the difference in speeds between the upper and lower levels and therefore causes the surface wind to veer (in the northern hemisphere) and strengthen, when compared with the night time mean surface wind. At night, during winter, over land, when a strong inversion has formed there will be little or no mixing between the air at 2000 ft and the air at the surface, indeed the limit of mixing may be at 500 or 1000 ft. This could well result in a fresh wind at 2000 ft (above the inversion) and a calm wind at the surface (below the inversion). When we consider the question of windshear we will see that this marked change of wind speed can lead to problems for aircraft which are taking off or landing. The second important factor is the speed of the surface wind itself. The stronger the wind, and of course the rougher the surface, the greater will be the depth of the mechanical turbulence mixing. As with thermal mixing the deeper the mixing layer the smaller the deviation between the 2000 ft wind and the surface wind. Of the two contributing factors surface heating is the most significant, giving rise to a distinct diurnal variation of wind velocity over the land, as shown.

Thus far we have considered the relationship between the 2000 ft wind (the geostrophic wind as we have considered it, but equally the gradient wind, should the isobars be curved) and the surface wind. We conclude by examining the differences in the wind at a higher level but still potentially within the friction layer. The 1500 ft wind can be taken as a good illustration of this effect. Over the sea the 1500 ft wind is likely to be close to the 2000 ft wind both by day and by night, since there is little thermal mixing and any mixing layer caused by turbulence is very unlikely to extend to 1500 ft. Similarly, over the land at night there is no thermal mixing and (since the surface wind speed will have decreased in speed from its daytime high) the turbulence layer is again


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unlikely to extend to 1500 ft. Over the land by day, however, the stronger surface wind and the thermal mixing from the warm surface will result in a mixing layer which could easily extend to 1500 ft. Assuming this to be the case, the 1500 ft wind over the land is likely to be close to the 2000 ft wind by night, but is likely to decrease in speed (and back in the northern hemisphere) as it mixes with the surface wind during the day.

Local Winds The preceding paragraphs have considered very much an idealised situation. It is now necessary to consider the way in which certain features, such as coastlines, villages and hills, can affect the flow of air in a localised situation. Sea Breezes During the day, the land surface temperature rises but the sea surface temperature remains almost constant. Due to this differential surface heating, the pressure above about 500 ft over the land rises, although the pressure at the surface is, for the moment, unchanged. There is now relatively high pressure above 500 ft over the land and air flows out to sea, a gentle drift of one or two knots. As air is being taken away from the land, the pressure at the land surface must now begin to drop slightly. Conversely when air begins to accumulate at above 500 ft over the sea, the pressure at the sea surface rises. With high surface pressure over the sea and low surface pressure over the land, the air at low level now flows from sea to land as a sea breeze, as illustrated.

In the fullness of time the sea breeze would turn to blow parallel to the coast line under the influence of geostrophic force. By the time that the geostrophic force exerts its full effect, however, the land is starting to cool and the sea breeze to diminish in strength. Sea breezes are most likely to occur under clear skies in the summer and with a slack pressure gradient to give otherwise light winds.


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In temperate latitudes, sea breezes reach a maximum speed of around 10 kt, although in tropical latitudes the speed may be as high as 20 kt. As a rule, sea breezes do not extend more than 10 to 15 miles on either side of the coastline, and the breeze is confined to very low levels, diminishing in speed rapidly above 500 ft to become negligible in most cases by about 1000 ft. From an aviation point of view the primary significance of sea breezes is that they can sometimes cause advection (sea) fogs to drift inland to cover coastal airfields briefly. Fortunately the high land surface temperatures, which caused the sea breeze, will normally soon disperse the fog. Convergence with the existing wind can result in convection cloud, creating a sea breeze front or possibly even triggering a thunderstorm if conditions are suitable. In some cases, because the sea breeze tends to be shallow, windshear can occur at coastal aerodromes. Land Breezes A land breeze is the breeze which blows from land to sea by night. It is effectively a sea breeze in reverse, since by night the ground will be colder than the sea surface.


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Katabatic Winds As a land surface cools at night the air in contact with it will also cool and consequently increase in density. When this happens on sloping ground the dense air will tend to flow down the slopes as a katabatic wind. The strength of the wind will depend upon the degree of surface cooling, and so the strongest winds are likely at night under clear skies, especially with a snow covered surface. A well known example of a vigorous katabatic wind is the Bora, an offshore wind blowing off the high ground on the northern shores of the Adriatic. The wind sets in suddenly and frequently reaches speeds of well over gale force, with gusts in excess of 100 kt.

Anabatic Winds An anabatic wind is the reverse of a katabatic wind, but the air moving up the slope by day will be travelling at a much more leisurely pace. Except near a coastline where the anabatic wind is augmented by a sea breeze, it is seldom of any significance.

Valley Winds

Valley winds are also known as ravine or funnel winds. Air coming up against a mountain range tends to flow around the edges rather than over top, especially if the air is stable. Where a valley passes through the mountain range the air will tend to flow along the valley, even if the wind has to change direction in order to do so. Where the valley is narrow, or converges, the windspeed in the valley will increase sharply due to the funnelling or canalisation effect. A change in the general pressure distribution which causes the free air wind to change direction by as little as 20 or 30° may well cause the valley wind to change direction by 180° since this wind is constrained to flow along the valley.


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The Mistral is a valley wind which blows down the Rhone valley as it passes through the Alps in Southern France. It is significant to note that the enhanced speed of the Mistral is apparent beyond the confines of the valley, persisting over the coast and into the northern Mediterranean. Fohn Wind If air is forced to rise over a mountain range, adiabatic cooling of the rising air will result. Initially the unsaturated air will cool at the DALR, however if the ridge is sufficiently high, the air will cool to its dewpoint. As the air continues to rise beyond this point condensation will occur and the air will now cool at the SALR. Cloud will form and precipitation in the form of rain or snow on the windward side of the mountain may result. Assuming that the air is stable on reaching the top of the ridge it will descend on the lee side. If precipitation has occurred on the windward side it will now be dryer and consequently the condensation level on the lee side will be higher than on the windward side. Although the air will heat initially at the SALR it will heat at the DALR from the condensation level downward. In consequence the air will heat for longer at the DALR on the lee side than it cooled at this rate on the windward side. Furthermore the air has deposited a high proportion of its moisture content as precipitation on the windward slopes. As a result a warm dry air blows beyond the ridge as a fohn wind.

The name originates in the Alps, however another example is the Chinook which blows in the lee of the Rockies. Berg Wind The berg wind is very similar to the Fohn wind, but differs in that it is not the height of the mountain that causes the temperature increase, but the long descent where the air warms at the DALR


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Standing Waves/Mountain Waves Under certain conditions an oscillatory motion of the air may occur once it has been forced to rise over a substantial ridge or mountain range. Such a motion is illustrated . It is important to appreciate that the waves and any associated cloud formations remain stationary with respect to the obstacle which triggered their formation, hence the name standing waves.

The conditions which favour the formation of standing waves or mountain waves are as follows: (i) A ridge of suitable dimensions, ideally with a gently sloping face on the windward side and

a steeply sloping face on the leeward side. A ridge height of 500 ft or more above the surrounding terrain may give rise to standing waves, if the conditions which follow are met.

(ii) A wind which is blowing perpendicularly to the ridge, plus or minus 30°, with little change of

wind direction with height. (iii) A windspeed in excess of 15 kt at the top of the ridge, and increasing with height. (iv) A marked stable layer, ideally an isothermal layer or an inversion, between ridge height and

a few thousand feet above the top of the ridge, with less stable air above and below. Mountain waves may extend for many miles downwind of the ridge, it can be up to 500 nm downwind of the Andes, however 50 to 100 nm is more normal. The vertical extent of standing waves is also considerable, on occasions extending well above the tropopause. The average wave length of mountain waves in the troposphere is in the region of 5 nm although in the extreme they may be much longer. A good estimate of the wavelength (in nautical miles) can be achieved by dividing the mean tropospheric windspeed in the region of the wave formation by seven, such that a 5 nm wavelength would be associated with a mean tropospheric windspeed of 35 kt. Where standing waves extend into the stratosphere the wavelength is likely to increase above the tropopause.


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The depth of oscillation of an individual standing wave is considered in terms of the double amplitude, which is the distance from trough to peak. In general the higher the ridge and the stronger the wind the greater the amplitude of the resulting waves. The most severe conditions are likely to occur when the wavelength of the wave coincides with the undulating terrain down—wind of the ridge. An average double amplitude is 1500 ft (giving vertical velocities in the order of 1000 ft/nm). An extreme case in SA might be a double amplitude of 2000 ft/nm, however in the USA double amplitudes of 20,000 ft (giving vertical velocities of 5000 ft/nm) have been recorded. In mountainous areas the wave formation may well be disturbed by the terrain downwind of the source ridge, alternatively the wave formation may be disrupted by changes in the total airstream. In either case the resulting wave breaking effect may result in transient but severe turbulence, which is difficult to forecast. The diagram previous shows the three distinct types of cloud associated with standing waves. Bear in mind however that, if the air is sufficiently dry, these characteristic clouds may not be present to act as a visual warning of standing waves. Alternatively, the standing wave clouds may be present but obscured by other cloud systems, particularly frontal cloud. Cap cloud may form on the windward side of the mountain in much the same way as with fohn winds. This cloud is frequently carried down the lee side of the ridge by the wave formation as a cloud fall or fohn wall. With sufficiently moist air, rotor or roll clouds will form in the rotor zones. They may appear as harmless bands of ragged cumulus or stratocumulus lying parallel to and downwind of the ridge, but in fact are rotating about a horizontal axis which is the centre of the rotor zone. Appreciate that the rotor zones are caused by the breakdown of the flow into violent turbulence and therefore roll clouds should be avoided at all cost, not only because of the turbulence but also because of the icing risk in the right (wrong) temperature band. The strongest rotor normally forms under the crest of the first wave down—wind of the ridge and normally at a level which is near or somewhat above the ridge crest. There are not normally more than two rotor clouds in the lee of the ridge. The wave motion of the air may produce lens—shaped or lenticular cloud in the crests of the waves. The cloud is forming as the air rises and cools through its dewpoint at the upwind end of the crest and dissipating as the air descends and heats back through its dew—point at the downwind end of the crest. There is very little opportunity for the condensed water droplets to encounter ice nuclei and therefore high concentrations of supercooled water droplets may give serious icing problems at temperatures as low as 30°C. Lenticular cloud normally appears up to a few thousand feet above the ridge height but may be seen at any level in the tropopause and perhaps even in the stratosphere. The outlines of lenticular cloud are normally smooth, often appearing as a stack of inverted saucers, however ragged edges to these clouds should be taken as a warning of turbulence, possibly due to wave breaking. We have yet to consider jet streams, suffice for the moment to say that jet streams produce turbulence and vertical windshears and that both can be greatly enhanced in intensity and extent when a jet stream (which is a fast flowing tube of air just beneath the tropopause) is found in conjunction with marked mountain/standing wave activity.


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The hazards associated with flight in standing waves are summarised below:

(i) Loss of terrain clearance in the troughs of the waves and rotor zones.

(ii) Severe turbulence in the rotor zones, especially in the first rotor zone.

(iii) Severe icing in roll clouds and lenticular cloud.

(iv) Large variations of airspeed and/or height, possibly to below the minimum safe altitude, downwind of the ridge.

(v) Increase in intensity of wind shear associated with a jet stream at high

altitudes. Whenever standing waves are forecast, or their presence is suspected, terrain clearance should be increased significantly, and flight in cloud should be avoided. If the route cannot be conveniently planned to avoid the area altogether, the track should be arranged to cross the ridge at right angles from the windward to the leeward side.

Rotor Streaming Rotor streaming. which should not be confused with rotor zones, occurs when very strong winds are blowing more or less perpendicularly to a ridge, but now the strong winds extend through a restricted depth when compared with the height of the ridge and diminish rapidly at some height above the ridge, thus preventing the formation of standing waves and associated rotor zones. In this event severe turbulence is likely downwind of the ridge, extending vertically from ridge height, to perhaps two or three times ridge height. In addition to the cap cloud which may or may not be present, rotor streaming may generate cumuliform cloud, which is likely to drift downwind, possibly dispersing as it drifts away.


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Thermal Winds A thermal wind is a component of the wind velocity at any given level which results from the mean temperature difference between two adjacent air masses. Take the concept down to a smaller scale and the drift of air from land to sea above a sea breeze is an example of the effect of a thermal wind component. Changing temperature leads to changing density which results in changing pressures at points above the surface, and it is the pressure gradient which results in wind. A simplistic but useful concept is of pressure being the weight of the column of air above a given point or level. If the column expands upwards due to increasing mean temperature there will be a greater depth of air above a stated level within the column than before. Consequently the pressure at this level will have risen, and an upper air high pressure area now exists.

Above illustrates how a pure thermal wind would occur. The low mean temperature in the left hand column has given low pressure at altitude whereas the high mean temperature in the right hand column has given high pressure at altitude. At 30,000 ft, a pressure gradient exists from warm to cold air. As the air starts to move under the influence of this pressure gradient, the geostrophic force will deflect the moving air to the left (in the southern hemisphere). Eventually the pressure gradient and the geostrophic forces will balance and the wind will now blow in the direction shown. Notice that, in this case, the surface pressure is equal beneath both columns and consequently the upper wind is entirely thermal in origin. From the above it can be seen that Buys Ballot’s law in respect of thermal wind component can be rewritten as stating that, if you stand with your back to the thermal wind component, low mean temperature is on your left in the northern hemisphere and your right in the southern hemisphere.


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The strength of the thermal component at any given level depends on the mean temperature difference between two air masses and the height band over which the temperature difference is considered (the greater the depth of air which is considered, the larger the thermal wind component becomes for any given mean temperature difference). In temperate latitudes a formula based on these two factors can be used to give an approximation of the strength of the thermal component: Speed of thermal Mean temperature The height of the wind component = gradient (0C per x layer considered 100 nm) (in thousands of feet) The above formula serves as a useful approximation only. Questions involving the use of this formula are not currently set in the examination. It does however serve to emphasise the important point that the greater the difference in mean temperature of adjacent air masses, and the greater the depth of air considered, the stronger the resulting thermal component. Jet Streams A jet stream is a ribbon of very fast moving air which occurs close to the tropopause. The ribbon itself may be many hundreds of miles long, some 200 nm across and 2nm’s deep. Jet streams are generally said to be present when the speed of air exceeds 60 kt, however speeds of 100 kt, occasionally 200 kt, and exceptionally 300 kt, may occur. The principal component of any jet stream is invariably the thermal component. When the geostrophic component is from approximately the same direction as the thermal component the speed of the upper wind will be even greater.

The polar front jet stream is the one which concerns us in Sth Africa. The polar front is discussed in depth later on. Suffice for now to describe it as a definite interface of polar air to the south and temperate air to the north. The mean position of this interface lies at a latitude corresponding to the north of Zimbabwe in the summer, but the Cape Coastal regions in the winter. It is disturbances along the front which gives us most of our bad weather in the form of polar front depressions and their associated warm and cold fronts.


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As we have now established the thermal wind component will be strong when a significant difference of mean temperature exists through a deep layer of air vertically, but over a short distance horizontally. A distinct interface between cold polar air and warm subtropical air qualifies admirably, hence the polar front jet stream. Since the environmental lapse rate will tend to become isothermal above the tropopause the depth of air which will give the greatest difference in mean temperature across the polar front extends from the surface to the tropopause. It is for this reason that the jet stream associated with the polar front occurs close to but just below the tropopause as illustrated.


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The following points are important to note in the above diagram. (i) The tropopause occurs at a lower altitude in the cold air than in the warm air, the change of

height occurring fairly sharply at the front itself. (ii) The interface (frontal surface) between the polar and subtropical air is sloping, with the cold

air undercutting the warm air. (iii) The jet stream is denoted by isotachs, which are lines joining points of equal windspeed. (iv) The core of the jet stream is located just below (normally less than 5000 ft below) the

tropopause in the subtropical warm air. (v) Since the wind is obeying Buys Ballot’s law, the cold air lies to the north of the warm air and

therefore the jet will be blowing from a westerly direction. (vi) The isotachs are most closely packed together on the cold or polar air side of the core of

the jet stream and it is for this reason that the polar side of the jet is the likely location of the most serious windshear and clear air turbulence.

(vii) Since a jet stream is often associated with fronts, the presence of cirrus cloud may serve as an indication of the presence of a jet stream, particularly at a warm front, however jet streams do not themselves generate cloud and frequently occur in clear air.

Over the Atlantic the movements of frontal systems means that position of the polar front jet stream can move north or south from its forecast position quite rapidly. Apart from very strong headwinds the major problem with jet streams is clear air turbulence (CAT). Whenever adjacent streams of air are travelling at different velocities turbulence will occur. Within a jet stream the speed of the air is changing rapidly with departure from the core, especially towards the polar air side of the core. This is important. To avoid the clear air turbulence the best course of action is to descend into the warmer air.

When CAT associated with jet stream activity is forecast a close eye should be kept on Doppler, INS or GPS drift and groundspeed indications. When either are changing rapidly you are approaching the jet stream and clear air turbulence is likely.


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The outside air temperature gauge can also give some warning. It is wise to strap the passengers and crew in, since CAT can be every bit as severe as the turbulence experienced in heavy thunderstorm activity. CAT is not only experienced with jetstreams, it can also occur above rapidly building cumuliform cloud, in sharp upper troughs (and occasionally in upper ridges), and in rotor zones and rotor streams which are devoid of cloud. Whilst it is unlikely that standing waves will give CAT as such (other than in the rotor zone), the presence of a standing wave can intensify the CAT associated with a jetstream flowing through the upper waves. Jet streams occur anywhere where a strong thermal gradient exists between air masses. The subtropical westerly jets which occur in each hemisphere between equatorial and subtropical/temperate latitude air masses at about 30N and 30S are similar in structure to the polar front jets. In the northern hemisphere summer are easterly jet stream also occurs at high levels near the equator.


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QUESTIONS 1. If there is no change in the pressure distribution, (southern hemisphere) the

surface wind towards midday will:

a) increase and back; b) decrease and veer; c) decrease and back.

2. The surface winds flow across the isobars at an angle rather than parallel to the

isobars due to:

a) surface friction: b) Coriolis force; c) the greater atmospheric pressure at the surface.

3. Winds at 5000 feet above ground level on a particular flight in the Southern

Hemisphere are north-easterly while most of the surface winds are easterly. This difference in direction is primarily due to:

a) a stronger pressure gradient at higher altitudes; b) stronger coriolis force at the surface; c) friction between the wind and the surface.

4. Wind speed and direction are usually in:

a) knots and degrees from true north: b) knots and degrees from magnetic north; c) miles per hour and degrees true north.

5. The force that acts at right angles to the wind causing it to flow parallel to the

isobars in the Southern Hemisphere is known as:

a) the geostrophic force. b) the coriolis force. c) the pressure gradient force.

6. The instrument used to measure wind speed is:

a) The hydrometer. b) The anemometer. c) The anegraph.

7. The winds which blow from opposite directions in the summer and winter in

certain regions of the tropics are called:

a) Trade winds. b) Monsoon winds. c) The doldrums.

8. A Fohn wind is hot and dry because:

a) Like a berg wind, it blows down slope. b) Much of its water vapour is lost on the windward slopes. c) Much of its moisture is lost on the leeward slopes.


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CHAPTER 4

AIR MASSES An airmass is a large body of air whose properties are horizontally uniform with regard to temperature and humidity and vertically uniform with regard to lapse rates. Its characteristics are dependant on its origin. CLASSIFICATION Polar (P) Tropical (T) Maritime (M) Continental (C) Warm (W) Cold (K) NOTE: The thermodynamically classification is compared to the surface over which it was moving. SUPERIOR AIR Air which originates in the upper atmosphere above permanent anti-cyclones. This air is continually sinking and in so doing, becomes warm, stable and very dry. FACTORS AFFECTING MODIFICATION OF AN AIRMASS

The characteristics of the area over which it moves after leaving the area of origin.

Speed at which it moves.

Diurnal variations e.g. a cold mass of air moving inland during the night will not be modified as much as during the day.

Mechanical influences that cause mixing of airmass.


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GENERAL PROPERTIES OF A COLD AIRMASS (POLAR) PM: UNSTABLE, SLIGHTLY MOIST - Good for Cumulus cloud and showers. - If this air moves over cold land, it becomes stable with low cloud, fog and

drizzle. PC: STABLE, VERY DRY Very little chance of cloud development. GENERAL PROPERTIES OF A WARM AIRMASS (TROPICAL) TM: STABLE, MOIST - If cloud does occur, it will be low with fog and drizzle. - If this air moves over very hot land, it will become unstable with convective

thunderstorms. TC: STABLE, DRY Nearly always cloudy. QUESTIONS 1. The cold polar air advecting out of the Antarctic is:

a) Moist if it has been moving rapidly over the Atlantic Ocean. b) Dry. c) Dry during the summer months and moist during the winter months.

2. Tropical Maritime Air moving over a warm land mass is most likely to produce:

a) Cumulonimbus cloud with good visibility. b) Stratus cloud with bad visibility and drizzle. c) Stable conditions with good visibility.


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CHAPTER 5

CLOUDS In this chapter the various means by which cloud forms and dissipates are discussed. You will learn to classify cloud by height and appearance. In the following section, dealing with Practical Meteorology, you will learn how to encode and decode those shorthand notations of cloud descriptions which are in common use (for example in the written form in TAFS and METARs, and in diagrammatic form on synoptic charts).

Cloud Cloud forms when condensation occurs. In the atmosphere hygroscopic nuclei are required in order for the water droplets to condense. With an abundance of such nuclei, for example particles of salt or sulphuric combustion products condensation can occur before the air has cooled absolutely to its dewpoint. The consequent difference between the condensation level and the saturation level is of academic rather than practical interest. At very low temperatures cloud forms by the process of sublimation. In this situation the air saturates at a sub zero temperature and the water changes state directly from vapour to solid ice crystal state. For this process to occur ice nuclei are required. It is because there are normally insufficient ice nuclei present in the atmosphere when the temperature is only at least sub—zero, and therefore water vapour is still being released at a fairly high rate, that supercooled water droplets occur. A supercooled water droplet is simply a droplet of water which exists in a liquid state at a temperature which is below 0°c, Cloud which is at a temperature of 0°c to –10°c will consist predominantly of supercooled water droplets.. Between –10°c and –40°c the ratio of ice crystals to supercooled water droplets will increase, whilst at temperatures below 40°c only ice crystals remain.


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In order for air to be cooled to its dewpoint (or frost point) one or more of the following processes must occur;

net loss by conduction to the cold surface of the earth loss of heat by radiation from the air adiabatic cooling due to ascent of the air

The first of these processes will result in the formation of dew or hoar frost or, with a little turbulence mixing, fog or mist. This is in effect cloud at the surface, however a freshening of the wind may cause the fog or mist to lift into the low cloud. Direct cooling by radiation from the air itself, or more correctly from the water vapour contained in the air, may contribute to the condensation process but is unlikely to be the sole cause of cloud formation. It appears therefore that, apart from the lifted fog situation, all cloud forms due to the adiabatic cooling which results from the vertical movement of air. This vertical movement may result from any one (or a combination of two or more) of the following;

turbulence orographic ascent convection widespread ascent, such as that caused by air converging at a warm, cold or occluded

front, or within a depression. There are basically two types of cloud, heap or cumuliform cloud, and layer or stratiform cloud. The type of cloud which forms when air becomes saturated depends upon the type of lifting process and stability of the air. If you have read and understood chapter 2 you should appreciate that unstable air will give cumuliform cloud, whereas stable air will give stratiform cloud. Cumuliform cloud is characterised by its marked vertical development. Clouds may be scattered with blue skies between, or may lie in a continuous line, as occurs at a cold front. Cumuliform cloud gives showery precipitation which is often heavy, and the risk of turbulence and icing. The greater the vertical development of the cloud, the greater are the risks inherent in flying through the beast. Stratiform cloud forms in more or less uniform sheets, often with clear air between the layers. The precipitation associated with this kind of cloud is more likely to be continuous and light or moderate rather than heavy. Severe turbulence is rare in layer cloud and the rate of ice accretion is normally low or moderate rather than severe.

Helpful Hint ! To work out the height of the cloud base, find the difference in the wet and dry bulb temperatures, and multiply the number by 410. You have now found the cloud base height above ground in feet.


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Cloud Classification Cloud is classified in two ways, according to the height of the base, and according to the characteristic appearance. The table at below gives the heights of the base of low, medium and high cloud.

The tables on the next pages give a generalised description of the various types of cloud formation, together with an outline of probable associated precipitation and significant weather. Cu, Cb and Ns are, by definition, low cloud, since the base height occurs below 7,000 feet. The first of the three, cumulus cloud, may well be contained within the first 7,000 feet of the atmosphere, especially in its fair weather form. Cumulonimbus and nimbo—stratus clouds, however, commonly straddle two or three of the height bands. It is for this reason that these clouds are commonly given their own sub—classification, as low cloud with a marked vertical extent. Prefixes are used to denote high cloud (cirro), medium cloud (alto) and rain bearing cloud (nimbo). Upper cloud does not extend much above the tropopause. As the height of the tropopause decreases towards the poles so do the tops of the upper cloud levels. You will already have encountered a cloud description which is not mentioned in the preceding tables, namely lenticular (or lenticularis) cloud which is often associated with standing waves. Another cloud description which is worthy of note is altocumulus castellatus cloud. This is medium cloud with a distinctive turret or tower like appearance, and often occurs in rows or lines. Castellatus cloud is associated with instability in the middle atmosphere and, like lenticular cloud, is worthy of note because of the possible high icing risk. Since altocumulus castellatus is indicative of middle atmosphere instability, its presence is often an indication of the likely development of thunderstorms. Finally, there are two other cloud types which occur at very high altitudes. The first is noctilucent Cloud, which is found at the 80 km level (260,000 ft) and is believed to consist of ice crystals. The second is nacreous cloud, which occurs at sunset at altitudes of between 20 and 30 km (65,000 and 98,000 ft). Because of their colouring, these clouds are sometimes known as mother of pearl clouds, in any event neither of these cloud types significantly affect the weather within the troposphere.

ATP MET Revised 02-2000

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Family or Group

Type or Genus

Abbrev. of

name

Composition

Usual form and a few Characteristics

Precipitation

HIGH

CLOUDS (5 to 13

km)

Cirrus Cirrostratus Cirrocumulus

Ci Cs Cc

Ice Crystals Ice Crystals Ice Crystals or water droplets

Fibrous, fine and thin, frequently resembles commas, casts no shadows. Uniform continuous thin white layer does not obscure sun or moon Small globules or ripples. Usually also Ci or Cs in vicinity.

None None None

MEDIUM CLOUDS

(2 to 7 km)

Altocumulus Altostratus

Ac As

Water, ice or snow particles Water and / or ice.

Rolls or globules in lines or patches, fairly thin, ± 90 to 250 m thick but able to cast shadows.Uniform rather amorphous layer or sheet, grey in colour. Sometimes thin but may be 3 to 5 km deep and then tends to become Ns.

Seldom, if any, only slight rain or snow Light or moderate rain or snow.

LOW CLOUDS (from the surface to

2 km)

Stratus Stratocumulus Nimbostratus

St Sc Ns

Water droplets Water droplets Water and ice particles, latter especially in upper layers.

Uniform, fairly dense, nebulous, sometimes with ragged base. Frequently hangs low enough to cover higher ground. Frequently occurs when mist or fog lifts also referred to a lifted fog Layer cloud with rolls or waves, usually dense but not deep, approximately 150 to 900 metres, elements larger than those of Ac Dense uniform layer, 1 to 6 km thick

Usually none, if any, only drizzle or light snow. Seldom Continuous rain, the name implies rain from cloud

CLOUDS

WITH VERTICAL DEVELOP-

MENT

Cumulus Cumulonimbus

Cu Cb

Water droplets Water droplets below, ice crystals in upper regions.

Heap clouds,flat bases, cauliflower upper parts, dense but attached clouds. Enormous cloud masses with tops like mountains or towers; the extreme crests are fibrous and often like an anvil.

Only localised showers when clouds in advanced stage Rain showers and / or hail, squalls and thunder.

ATP MET FLIGHT TRAINING COLLEGE ATP DOC 9 Revision 1/1/2001 Version 5

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Turbulence Cloud The formation of a turbulence (or mixing) layer as air moves across the surface was discussed in a previous chapter. Below diagrammatically shows the effect of vigorous mixing within a stable layer of air, the ELR of which was originally 1C/1 000 ft, extending from the surface to 3000 ft.

The air within the friction layer has become thoroughly mixed and, due to adiabatic cooling and heating, the ELR has been modified, changing from its original stable rate to, in this case, 3C/1 000 ft. Rather than complicate matters, it has been assumed in this example that the mixing within the turbulence layer has not resulted in the air being cooled to its dewpoint, and the subsequent formation of cloud. Suffice to say that, if the air had been cooled to its dewpoint due to mixing, the ELR above the condensation level would have tended towards the SALR, whilst the ELR below the cloud would have tended towards the DALR. The effect of mixing is always the same; the ELR tends towards the appropriate adiabatic lapse rate. The more thorough the mixing the greater will be the modification of the original ELR towards the adiabatic rate. Note the marked inversion which characteristically occurs at the top of the mixing layer.


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The mixing process and the subsequent redistribution of heat within the layer is assumed to be self contained, so that heat is neither gained nor lost by the process. The mean temperature in the layer is the same after mixing as it was before. The steepening of the lapse rate therefore results in the warming of the air in the lower levels and cooling near the top of the layer. The moisture contained in the air within the turbulence layer will become evenly distributed throughout the layer due to mixing. If the air contains sufficient moisture, the mixing and cooling may cause the air to become saturated within the layer, and cloud will form from the condensation level to the top of the mixing layer. Below shows such a layer of cloud. Note that the top of the cloud will be flat, the vertical motion of the air being effectively arrested by the inversion which has formed at the top of the mixing layer.

The cloud so formed will rarely exceed three or four thousand feet in depth. The base height of the cloud will depend on the depth of the mixing layer, which is itself dependent on the windspeed and the nature of the surface, and on the moisture content of the air. The cloud will either be stratus (with lighter winds and moist air) or stratocumulus (with stronger winds and drier air). Turbulence cloud will occur when the following conditions are fulfilled:

The turbulence is active enough to cause the ELR within the mixing layer to tend towards the DALR.

The air is sufficiently moist, so that saturation occurs within the layer, once mixing has

occurred.

The ELR above the mixing layer is stable (otherwise the air will continue to rise and cumuliform cloud will result).


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Turbulence cloud can only form when the surface air is sufficiently humid. A cold surface will obviously serve to raise the relative humidity of the air in contact with it. Over parts of SA this type of cloud often forms in the late evening when turbulence persists whilst the ground cools rapidly, raising the relative humidity of the air. For the same reason, the base height of existing turbulence cloud will often lower during the evening. Alternatively, turbulence cloud may form after sunrise following a clear night with low surface temperatures, although in this case the cloud is unlikely to persist once the surface temperature rises, however, when the cloud layer is thick, surface heating will be slow and cloud dispersal delayed. Heavy precipitation is unlikely from turbulence cloud, however a sufficiently thick layer of stratocumulus formed in this way may give drizzle or light flurries of snow with sufficiently low temperatures. Flight conditions which are likely when flying in or near turbulence cloud are summarised below:

Above the cloud flight will be smooth in the stable air and visibility will be good.

Within the cloud light or occasionally moderate turbulence is likely, visibility will be poor and above the freezing level (which will be modified by the mixing within the layer) airframe icing will occur.

Below the cloud the light or occasionally moderate turbulence will persist and visibility

will be poor due to the dust which is lifted by the mixing and subsequently trapped below the inversion.

Convection Cloud Turbulence cloud derives its name from the fact that it is the turbulence which causes the mixing which gives rise to the cloud. Similarly convective cloud is so called because the trigger action is convective. With this type of cloud the vertical extent is dependent very much on the ELR, possibly from the surface to the tropopause. The point in question in the preceding sentence is the point at which the surface temperature will increase to a value such that the adiabatic line passes to the right of the upper air inversion. Now the cloud development is rapid and a point of neutral stability may not be reached, in the extreme, until the isothermal layer above the tropopause is reached. In this event it is probable (in temperate latitudes) that the top of the cloud will occur at a height where further condensation or sublimation is inhibited by the lack of water vapour in the very cold air, rather than at the point where neutral stability is achieved. It should be appreciated that, in this case, turbulence will be experienced above the cloud, since the air is still rising.


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The instability illustrated above has occurred because of convection due to insolation. The resulting cumuliform clouds necessarily form only over the land and only by day. Convective cloud may however form due to advective convection. If cold air moves over a warm surface, the air at the surface will heat advectivelv. If this advective heating is sufficient, convective instability will result. The clouds thus formed may occur over land or sea and by day or night. The height of convective cloud is again primarily governed by the depth of the instability layer. If this layer is only a few thousand feet thick, then fair weather cumulus will develop. If instability exists through 10,000 feet or even 20,000 feet then towering cumulus or cumulonimbus clouds will form, given that the air is sufficiently moist. The marked instability associated with cumuliform cloud of marked vertical extent gives rise to strong vertical currents of air; to the formation of large water droplets; and to the characteristic heavy shower activity. The composition of a cumuliform cloud will be liquid water between the condensation level and the zero degree isotherm. Between 0C and -10C the cloud will consist almost entirely of supercooled water droplets, giving significant airframe icing problems. Between -10C and -40C the cloud will consist of a mixture of supercooled droplets and ice crystals with the proportion of ice increasing with increase of altitude and consequent drop in temperature. With temperatures below -40C the cloud is composed entirely of ice crystals and now the column like appearance of the cloud often breaks down. The ice crystals at the top of the cloud tend to drift downwind giving the anvil which is often seen at the top of towering cumulus and cumulonimbus. It is possible that a cumulonimbus will become an active thunderstorm cloud, and thunderstorms are discussed in a following chapter.


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Orographic Cloud The causes of orographic cloud formation are, generally speaking, the same as for turbulence cloud, however the effect of the surface turbulence is magnified. It might be imagined then that orographic cloud will be predominantly stratiform in nature. This is often the case, however a greater depth of air is affected and consequently the lapse rates and moisture content of the air at greater heights must be considered. Consider the behavior of air which is flowing perpendicularly to a ridge rising 3000 feet above the general surface level. The surface air will be forced upwards through 3000 feet, similarly the air which was at 3000 feet will be physically lifted to 6000 feet. Here then is a case where a whole mass of air, rather than an isolated pocket or parcel of air, is physically lifted and of course cools adiabatically. This will alter the ELR of the air within the lifted layer, possibly causing a previously stable air mass to become unstable. Assuming that the air is moist and that saturation occurs during the lifting process, the state of stability of the lifted air will determine the type of cloud which forms. If the air is stable after lifting the cloud will be stratiform in nature, If the air is unstable after lifting then cumuliform cloud will form, the depth depending on the thickness of the instability layer, and the amount of moisture present in the air. A Fohn wind (the warm dry wind on the leeward side of a ridge) results from orographic cloud formation. The cloud in this case is stratiform; the air is stable at the summit, which is why it readily flows down the leeward side. It is the loss of moisture as precipitation falls out of the cloud which gives the characteristically dry air downwind of the ridge.

By comparison the violent thunderstorms which sometimes occur over the Alps in Europe are another example of cloud which is predominantly orographic in nature. Here the air is most definitely unstable after lifting. Frontal Cloud The cloud associated with frontal systems is discussed in a subsequent chapter. Suffice for now to say that the gentle uplift of air associated with warm fronts tends to give predominantly stable conditions and consequently a wide band of stratiform cloud. The steeper interface of a cold front gives a more pronounced uplift of the warm moist air ahead of the front and now cumuliform cloud is the norm.


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QUESTIONS 1. Clouds formed by turbulence in stable conditions are usually of the genus:

a) Cu; b) St or Sc; c) Ac.

2. The suffix ‘nimbus’ used in naming clouds, means:

a) clouds with extensive vertical development: b) an accumulation of clouds; c) rain clouds.

3. Mist or dew always forms when:

a) water vapour in the atmosphere is present; b) the dew point and temperature are equal; c) the air is stable.

4. The outstanding characteristics of the weather in the tropics are:

a) large Cu and As clouds with heavy rain showers; b) Ns over a wide area with continuous rain’ c) Cu and As clouds with light showers.


a) The temperature/dewpoint spread can be used to determine cloud base. b) Relative humidity is the ratio of air to water. c) Vapour pressure is high when relative humidity is low.

6. The type of cloud usually caused by air rising over a hill is:

a) Cirrocumulus. b) Altocumulus. c) Stratus.


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CHAPTER 6

VISIBILITY Visibility is a measure of the transparency of the atmosphere. More practically it is a measure of the distance at which significant objects or features can be seen and recognised. By definition it is the maximum horizontal distance in a particular direction at which a dark object of certain dimensions can be seen against a light background such as the horizon sky by an average observer. When visibility varies with direction, the lowest value is measured. It is often possible to see lights, or shiny objects reflecting strong sunlight, at distances which are beyond the stated visibility, especially if they contrast with their surroundings. Visibility reported at night is that value which would be given by day in the same conditions of transparency of the atmosphere. Lights of known intensity are observed, and an allowance made for that intensity. The range at which the light can be seen is thus converted into equivalent daytime visibility. Meteorological visibility as defined and discussed above has serious limitations as far as pilots are concerned. Haze, mist and fog all tend to be layered, so that visibilities at different levels may be very different. Furthermore light coloured objects won’t be seen against a sky background until the range is considerably less than the published visibility (why is it that so many gliders are painted sky blue?). Flight visibility (which is relevant when assessing VFR criteria) is defined as being the visibility forwards from the flight deck. Further problems exist when looking obliquely at the ground, through layers of fog, mist or haze, and these situations are discussed later on in the chapter Obscuring matter which will reduce the transparency of the atmosphere, and therefore visibility, may be classified as follows:

Fog. Mist. Cloud. Precipitation. Sea spray. Smoke. Sand. Dust. Fog

FOG Fog is cloud at ground level. It exists, by definition, if the surface horizontal visibility is reduced to less than 1000 metres due to the presence of water droplets which are held in suspension in the air. Fog normally forms where conductive cooling from a surface below the dew point temperature of the air occurs. Radiation and advection fogs are attributed to this method of cooling. Fog can also form if additional moisture is supplied to the surface layer of air. This occurs when precipitation is followed by evaporation which increases the relative humidity in two ways. Firstly the water vapour content of the air is increased as the moisture evaporates,


60

and secondly the heat energy required for the process of evaporation is taken from the air, thereby lowering the temperature. Mist Mist is thin fog The definition is as for fog except that the visibility is now 1000 metres to 5000 metres. Haze Haze is defined as a reduction of surface horizontal visibility, but not to below 1000 metres, due to solid particles held in suspension in the air. Were the visibility to drop below 1000 metres, the nature of the obscuring matter would be specified, as, for example, smoke or dust. Smoke Fog With smoke fog the visibility is reduced to less than 1000 metres, with the obscuring matter being a combination of water droplets and solid particles which are produced as a by product of combustion. Radiation Fog Radiation fogs will only form over land since a significant diurnal variation of surface air temperature is a prerequisite of this kind of fog. Remember that, under clear skies and with light winds, the diurnal temperature graph follows a downward curve from around 1400 LMT to dawn the following day. If the air is sufficiently moist, the cooling air will pass its dewpoint and the moisture will condense out as visible water droplets.

The ideal wind speed for creating the turbulence necessary to support the water droplets is two to eight knots. Exceptionally, there may be sufficient turbulence to form and maintain fog even when the surface wind is reported as calm. At wind speeds of 10 kt and above fog will either tend to disperse, by mixing with the drier air above, or lift to form low stratus. The requirements for radiation fog to form are therefore: (i) Moist air. (ii) A land surface. (iii) Clear skies. (iv) Light wind. (v) Hygroscopic nuclei. The longer the night and the lower the temperature the more likely is the formation of radiation fog, which is therefore most frequent in the Southern African regions in late autumn, winter and early spring, although radiation fog may occur at other times. Radiation mist is a fairly common feature around sunrise on a bright summer morning.


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Radiation fogs are most prevalent over low—lying ground, especially where there is a moisture source such as a marsh, lake or river. Fog which forms on hillsides will tend to drift downwards under the influence of katabatic drainage. The synoptic conditions favouring the formation of radiation fogs are anticyclones, ridges and cols, which tend to provide the necessary clear skies and light winds. Radiation fog may form in the late afternoon, or at dusk, or at any time during the night. The fog will form whenever the ground cools the air in contact with it to below the dewpoint, providing that the wind speed is within the critical limits.

Radiation fog frequently forms rapidly shortly after sunrise following a night with clear skies and calm wind conditions. Remember that two to eight knots of wind are required to give radiation fog. Since the conditions have been calm through the night, the condensation has caused a heavy dew, however at sunrise the wind will tend to strengthen. Add to this the thermal mixing as the sun heats the ground and excites the surface layer of air and, quite suddenly, a radiation fog can form. Radiation fogs normally disperse in the opposite manner to which they form. After sunrise, the sun’s rays penetrate the fog and heat the surface. The surface warms the air to above its dew point and the fog evaporates into the air as vapour. If the fog is too thick, however, or if a layer of cloud covers the sky once the fog has formed, it is likely that dispersal will be delayed. Indeed, the small amount of thermal turbulence and the slight diurnal increase in windspeed experienced under these circumstances may serve to thicken the fog rather than disperse it. Once this situation has developed, the fog is likely to persist until there is an increase in surface wind velocity to greater than 8 kt, or a change to a drier air mass. Radiation fog rarely exceeds a few hundred feet in depth, which is the usual depth of the mixing layer caused by light winds.


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Advection Fog The cooling processes involved in advection fog is provided by the movement (or advection) of warm moist air over a cold surface, the temperature of which is below the dewpoint of the air. Advection fog, unlike radiation fog, can and does form readily over the sea as well as the land. Hill Fog Hill fog is simply low cloud which is covering high ground. The presence of the high ground may or may not have contributed to the presence of the cloud. Frontal Fog Both radiation and advection fogs are described as air mass fogs since they depend on cooling taking place within an extensive and more or less uniform mass of air. By contrast, frontal fog occurs at the surface position of the interface between two adjacent air masses. Frontal fog may form in one of two ways. The frontal cloud may come down to the surface as the front passes a given point. This is more likely to happen over high ground. Alternatively the increase in moisture due to the frontal rain may cause saturation resulting in condensation. This type of fog is most likely to occur at a warm front or warm occlusion. Steam Fog

As you can see steam fog forms when cold dry air blows over a warmer surface, that being either a warm water surface, or warm moist land such as a swamp or marsh. It can also form over the arctic regions over water, and is caused by very cold very dry air from the ice pack blowing over the relatively warmer ocean surface. A relatively light wind is required to form steam fog inland, but a steam fog over sea may form and actually increase in depth and distribution with a strong wind (ie >15kt).


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Visibility and Flying As already discussed the meteorological (horizontal surface) visibility given by ATC to a pilot may have little relevance, especially in foggy, misty or hazy conditions. If a mist or haze layer lies well below the aircraft, the distance at which the ground will be visible will increase with height. Conversely, as the aircraft descends into the top of a shallow mist or fog layer, this distance will decrease markedly. Below illustrates a situation where the depth of fog layer exceeds the distance which the pilot can see through the fog. The aircraft at the highest position is above the fog layer and the ground is not visible from the aircraft. At position where the aircraft is turning onto final, the ground is visible but not the runway.

A different problem exists when the visibility through the fog exceeds the vertical depth of the fog. In this case the pilot might be reassured, since the runway is clearly visible on the downwind leg. As seen, however, the runway will not again become visible until a very late stage of the final approach. It is unwise to attempt a visual approach to land if this involves descending into an obscuring layer of fog, mist or haze without visual contact with the runway. Visibility is better looking out of sun rather than looking into sun in the situations previously described. This is principally because of the glare factor. Conversely, looking into moon will give better visibility than looking out of moon, principally because of the better contrast. Visibility in Cloud Visibility in cloud varies with the cloud type. In cirrus, the visibility may exceed 1000 metres whilst in cumulonimbus the visibility may be no more than 10 metres. Visibility in Precipitation Visibility in rain depends on the size of the raindrops and their concentration. Heavy tropical rain may give a visibility of only 100 metres, whilst heavy rain in temperate latitudes may limit


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the visibility to 1000 metres. Moderate rain gives a visibility of between 3 and 10 kilometres. In drizzle, the visibility is often seriously reduced, especially if mist is also present. Heavy rain can affect a pilot’s perception of distance from the approach or runway lights by diffusing the glow of the lights and causing them to appear to be less intense and therefore more distant than is actually the case. Alternatively rain on the windscreen can cause runway lights to bloom and double their apparent size, causing them to appear to be much closer than is actually the case. A heavy rain shower moving towards an aircraft making a visual approach to land can cause a shortening of the pilot’s visual segment, which is that distance along the surface visible to the pilot over the nose of the aircraft. This may produce an illusion that the nose of the aircraft has pitched up. The natural response should be guarded against, since it would involve lowering the nose of the aircraft and/or reducing power. Snow reduces the visibility considerably, typically down to 1000 metres or less in moderate snowfall but down to between 200 metres and 50 metres in heavy snowfall. Drifting snow will give similar visibility to falling snow. Visibility in Smoke The incomplete combustion process in a fire, be it domestic, industrial or natural, introduces solid carbon particles into the atmosphere. The larger particles will soon settle on the surface, however the smaller particles will remain in suspension in the atmosphere. The resulting reduction in visibility will depend on the rate of introduction of the combustion waste into the atmosphere, the rate of dispersal, and the distance of the observer from the source of pollution. The problem will be the greatest in, and downwind of, industrial regions. The dispersal of smoke may occur either vertically, horizontally, or both. With an unstable atmosphere, the obscuring matter will be carried upwards. Conversely, with an inversion the obscuring matter will be effectively trapped beneath the inversion and the visibility will be poor. In such conditions, the top of the inversion layer will be clearly defined by a marked improvement in visibility in the air above. In extreme cases a haze horizon will be apparent when looking down on the inversion from above at a shallow angle. The wind is the principal factor governing the horizontal rate of dispersal of smoke, the stronger the wind, the better-the visibility. When carbon combustion waste is introduced into an inversion layer already affected by mist or fog, smoke fog results and the visibility is now seriously reduced. Such smoke pollution occurs in industrial regions and increases the frequency, density and persistence of the fog layer. Visibility in Dust or Sand Dust and sand is raised into the atmosphere by the wind. Larger sand particles will normally only rise to twenty or thirty feet, giving the classic dust storms of North Africa and fog like levels of visibility. Dust storms, or Haboobs, frequently occur in the Sudan in the gusty winds under cumulonimbus clouds. Air mass dust storms, again prevalent over North Africa, present a very much less localised condition than the Haboob. Now smaller dust particles are carried up to three or four


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thousand feet, and the visibility may be reduced to 300 metres or less over hundreds of square miles.

Visibility in the Atmosphere Visibility in the horizontal plane generally increases with altitude. Both cloud amount and concentration of solid particles decrease with height. There are, however, other factors which must be considered when assessing the range at which another aircraft may be seen in flight, and these include: (i) Size, colour and illumination. (ii) Relative speed between target and observer. (iii) Transparency and cleanliness of windscreens and the field of view

offered to the pilot. (iv) The tendency of the human eye to adopt a short focal length in the

absence of external visual references. Finally, it should be appreciated that even with good in-flight visibility, the absence of a natural horizon may lead to serious disorientation problems. Runway Visual Range During a take—off run or when approaching to land the pilot requires information concerning the distance at which he may expect to see the runway markers or the runway lights as an aid to visual orientation. An assessment of meteorological visibility is, by definition, of limited value to the pilot under these conditions. One way in which the pilot is provided with a more pertinent assessment of the visibility is by passing him the Runway Visual Range, or RVR. RVR is defined as the maximum distance at which the pilot may expect to see the runway lights or runway markers, during a take—off or landing ground roll, from a point five metres above the touchdown point. RVR may be assessed by an observer stationed 76 metres from the centre line of the runway, abeam the touchdown point. The observer sights and counts (in the direction of landing) The numbers of the runway markers, runway lights or special reference lights positioned at known intervals that he can see. Using tables, this number is converted to RVR and passed to the pilot. Using this manual system, RVR is assessed and passed to the pilot whenever the meteorological visibility is less than 1500 metres. Instrument Runway Visual Range systems (IRVR) are used at major airports to assess RVR automatically using transmissometers. These instruments measure the atmospheric opacity along the runway using a beamed light source of known intensity which is shining at a photo—electric cell some four feet away. A problem arises with IRVR in that the fog may be patchy and an IRVR meter measures localised visibility. This problem is overcome by positioning one instrument at the touchdown end of the runway, one at the mid—point, and one at the upwind end of the runway, IRVR reports are passed when the visibility falls below 1500 metres, or when the observed IRVR is at or below the maximum assessable value for the equipment in use, or when shallow fog is forecast or reported. Between zero and 200 metres the RVR is reported in steps of 25 metres; between 200 and 800 metres in steps of 50 metres; and between 800 and 1500 metres in steps of 100 metres.


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QUESTIONS 1. Advection of moist air over a gradual upslope can be expected to:

a) produce low stratus and fog; b) produce nimbostratus or altostratus; c) cause cumulus and thunderstorms.

2. Which conditions favour the formation of radiation fog:

a) warm moist air over low, flat areas on clear, calm nights; b) warm, moist air moving over a cold surface; c) cold air moving over a warm surface.

3. The most frequent type of surface based temperature inversion is that produced

by:

a) the advection of colder air under warm air, or the advection of warm air over cold air;

b) terrestrial radiation on a clear, relatively still night; c) wide-spread sinking of air resulting in heating by compression.

4. Moist stable air flowing upslope can be expected to:

a) produce low stratus or fog; b) produce nimbostratus or altostratus; c) cause cumulus and thunderstorms.

5. The 0500 METAR from an airport indicated no clouds with a temperature dew point

spread of less than 20 C. At sunrise you can expect:

a) fog at the airport; b) thunderstorms at the airport; c) rain at the airport.

6. What types of fog depend upon a wind in order to exist:

a) advection and upslope fog; b) steam and downslope fog; c) Tropical air fog.

7. Under which conditions does advection fog usually form:

a) moist air moving over colder ground or water; b) a light breeze blowing colder air out to sea; c) a land breeze blowing a cold airmass over a warm ocean current.


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8. What are the characteristics of advection fog, radiation fog and steam fog:

a) Radiation fog results from cooling the air to its dew point, while advection and steam fog require the addition of moisture to the air near the surface through evaporation.

b) Advection fog deepens as wind speed increases up to 20 knots, while steam fog requires calm or very light wind, and radiation fog forms when the ground or water cools the air by radiation.

c) Radiation fog is restricted to land areas, while advection fog is most common along coastal areas and steam fog forms over a water surface.


a) In mist the visibility is less than 1 km. b) Radiation fog occurs after the horizontal transfer of heat. c) If the wind speed is greater than 10 kts, radiation fog will dissipate and change

into Fs, St or Sc. 10. The fog type most common along the West coast of South Africa is:

a) Advection fog. b) Radiation fog. c) Guti fog.

11. With regard to the depth of fog:

a) Advection fog is likely to be deeper than radiation fog. b) The depth of the fog bears no relation to its type. c) Radiation fog is likely to be deeper than other types of fog.


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CHAPTER 7

PRECIPITATION The moisture released from cloud is termed precipitation and may take the form of water in either its liquid state, rain, or its solid state, snow or hail. Precipitation is classified in meteorology according to its size, shape, composition, and duration. When water vapour condenses onto hygroscopic nuclei within a cloud the droplets so formed will initially be 0.02 mm or less in diameter. The smallest droplets reaching the surface as rain will be of approximately 0.2 mm in diameter and the largest approximately 5.5 mm in diameter. Obviously then the condensed water droplets must have grown in size within the cloud before falling out as precipitation. The mechanics of growth are described in the two theories of precipitation. Coalescence Theory This theory suggests that rain drops form because minute condensed water droplets collide with each other and therefore increase in size. The problem with this theory is that, since the upcurrents in a given cloud will be fairly uniform, all water droplets will tend to travel upwards at a constant speed and therefore coalescence (or collision) is unlikely to readily occur. Once a differential in water droplet size is established the theory becomes easier to swallow, since now the heavier droplets will travel upwards at a slower rate than the smaller droplets, and a chain reaction is now established. The coalescence theory offers the only explanation as to how precipitation forms in cloud which is wholly at a temperature of above 0°C. The Ice Particle Theory The ice particle theory, otherwise known as the Bergeron process, suggests that ice particles must be present in the upper part of the cloud before precipitation can occur. If the cloud is forming up to heights where the temperature is below freezing, some of the water droplets carried up will freeze on to ice nuclei. The proportion of frozen droplets will increase towards the top of the cloud as the temperature drops further below 0°C. According to the theory in question the ice crystals grow in size due to sublimation and to collisions with supercooled water droplets. Eventually the ice crystals become too large to be supported by the upcurrents and they begin to fall through the cloud. During the descent they become larger still, due to further collisions with liquid water droplets. Depending on the temperature the precipitation which started its descent from the top of the cloud as an ice crystal will leave the base of the cloud either as rain or as snowflakes. The coalescence theory offers no explanation for the formation of snowflakes. The ice particle theory offers no explanation for the formation which occurs from clouds which are wholly at temperatures above 0°c. It would therefore appear that the two theories are complementary. The size of raindrops, however formed, is proportional to the upcurrents present in the rain bearing cloud. This is one reason why not all clouds give precipitation. If the upcurrents are very weak the water droplets falling out of the cloud will be very small, and will totally evaporate within the unsaturated air beneath the cloud before reaching the surface.


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Water droplets, or ice particles, will be held in suspension in the cloud until they grow to such a size that the terminal velocity of the raindrop exceeds the velocity of the upcurrent attempting to support it. The table below shows the relationship between droplet size and terminal velocity. Note that the maximum droplet size quoted in the table is 5.5 mm, and that a vertical upcurrent of 30 feet per second is required to hold a droplet of this size within the cloud. Vertical velocities of this magnitude are not uncommon in large cumulonimbus cloud. Stronger upcurrents will not result in larger water droplets falling out of the cloud as precipitation, since these larger droplets will break up due to air resistance during their descent. Diameter

(mm)

0.02

0.1

1

2

3

4

5.5 Terminal Velocity (ft/sec)

0.04

0.8

12

21

26

28

30

Rainfall is classified as drizzle if the size of droplets reaching the surface is less than 0.5 mm. As the size of the droplets increases moderate rain is said to be falling, whilst heavy rain occurs when the droplets are approaching maximum diameter (5.5 mm). There are no clearly defined parameters to distinguish between the various categories, although drizzle is often defined as being rain which is so light that it causes no appreciable pattern to be formed when it falls upon a still water surface. The duration of rainfall is important. Continuous rain is self-evident, however there is a distinct difference between intermittent rain and showers. With intermittent rain, the sky remains cloudy between periods of rainfall, whilst with showers the sky clears between the periods of precipitation. If precipitation fails in the form of ice (other than hail) three forms are classified, and these are roughly related to size: (i) Small grains of opaque white ice, normally less than 1 mm in diameter, form granular

snow. (ii) Crystals in the form of needles, normally about 2 mm long, are known as ice needles. (iii) Larger conglomerates of crystals which appear to be opaque and feathery in

composition form true snowflakes. Snow rarely reaches the surface if the temperature is much in excess of +4°C. This is because of the large surface area in relation to the volume of ice present, and to the low terminal velocity.

Sleet occurs when rain and snow fall together, or alternatively where snow partially melts as it falls. Hail is again precipitation in the form of ice. Soft hail consists of white opaque pellets rarely exceeding a few millimeters in diameter. It falls predominantly from cumuliform cloud in shower form during cold weather.


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True hailstones, or hard hail, are hard pellets of varying sizes, frequently with a structure of concentric layers of alternating clear and opaque ice. Hard hail is normally associated only with instability cloud of considerable depth. The ice particle theory of precipitation formation suggests that an ice crystal initially forms the core of any precipitating matter. With hail it is believed that, in the upper parts of the cloud where there is a scarcity of supercooled water, the ice particle grows in size only slowly. Here air is trapped as a new layer of ice forms around the original ice particle giving an opaque appearance to the ice layer. The hail stone now descends into the lower cloud where there is an abundance of supercooled water. Another layer of ice forms rapidly. Without trapping any air, and so this layer of ice will be clear or transparent. Should the magnitude of the up draughts now increase the hail stone will be carried upwards once again and the whole process repeated. In the case of a large cumulonimbus cloud in temperate latitudes the building process may be repeated often enough to give hail stones approaching 1 cm in diameter. At lower latitudes hail stones the size of golf balls occur, in South Africa stones of this size can and do devastate acres of crops in minutes, and cause considerable structural damage. In equatorial regions hail occurs only rarely at surface levels, because of the higher ambient temperatures which melt the hail before it reaches the surface.


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CHAPTER 8

FRONTS & PRESSURE SYSTEMS Fronts As we have just seen in, the development and movement of large homogenous air masses dominate the weather over large areas of the surface of the Earth. These carry with them the thermal and humidity characteristics of their source region, although they will be modified (especially in the lower levels) by the surface over which they are advected. It is inevitable that these large air masses will come together as they move over the surface of the Earth. It is equally inevitable that, when they meet they will have very different thermal, stability and humidity characteristics. At first the ‘zone of meeting’ will cover a long distance but as the air masses move towards one another, the transition from one to the other will become sharply marked. This sharp zone of transition is called a front. The front so formed has a profound influence on our lives. It is on fronts that our most frequent weather systems (frontal depressions)are generated. We will look t these in detail in a later chapter By the end of this chapter you will be able to describe the process of formation, the associated structure, movement, aviation weather (including local influence of the terrain) and wind-conditions associated with: - • warm front • cold front • occluded front • troughs and • weather behind the cold front Nature of A Front In the preceding chapter we saw that different air masses adjoin each other. The transition between them can be gradual but can also be more or less sharp. The zone in between them is called frontal zone and, where the transition is well defined (or sharp) the imaginary transition surface is called the frontal surface. The frontal surface is always inclined and forms when the lighter warm air slides up over the heavier cold air, which forms a wedge below the warm air.


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We talk about a front when the frontal surface intersects the ground surface. It is the front that is forecast by the meteorologist after having conducted an analysis of the surface weather charts. The frontal surface is not just a narrow line but is actually a zone of transition from one air mass to the other, the frontal zone. Its width varies between 60 - 90 miles at upper levels, while 30 - 50 miles are normal values at the ground surface. The slope of the frontal zone depends on the motion of the air masses and the difference of temperature (density) between them. When the two air masses are drawn together and the contrast between them increases the front forms and we talk about ‘frontogenesis’. The opposite is called frontolysis. The front concept was introduced in 1918 by J. Bjerknes and was further developed by Bergeron, among others, in something called ”the Bergen school”. At this time the number of observations was small and the quality mostly poor, at least as far as the upper levels were concerned, so the Bergen school created an idealised model of a frontal system. Observations have gradually improved, but it was not until the 1960´s that the front models were modernised. With satellites constantly taking pictures and Doppler radar dissecting cloud systems, new front models have been introduced, much owing to an Englishman by the name of Browning. The fact that two air masses adjoin each other does not necessarily mean that there is a well-defined front between them. Great contrasts in temperature and/ or humidity over a short distance are also required. Such sharp contrasts do not arise suddenly, but several processes are in progress during a longer period of time (day /days). The frontogenesis is a fairly complicated process and is outside the scope of this text. Here we will confine ourselves to illustrating a weather situation, where the wind pattern sharpens the temperature contrast between two air masses and cause a frontogenesis. In the chapter on ”Pressure systems” is there a simplified description of the development of an extra-tropical cyclone along the Polar front.


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Fronts, Definitions Warm front: Warm air displacing cold air Cold front: Cold air displacing warm air Occluded front: Forms when the cold front overtakes the warm front Warm front occlusion: The air behind the occluded front is the warmest Cold front occlusion: The air ahead of the occluded front is the warmest Stationary front: Essentially no movement across the frontal zone

We will return to the different fronts further on in the chapter. On the weather charts the various fronts are marked thus:

The warm front is red

The cold front is blue

The occluded front is violet

The stationary front is marked as shown and is divided into red and blue parts.

Frontal characteristics The slope of a frontal surface may be rather easily illustrated by means of a vessel with oil and water separated by a plate of glass. Oil is lighter than water and when the glass plate is removed a pressure gradient force, resultant from the differences in density, will press the water in under the oil, and we get a situation showing the slope of the frontal surface, as illustrated in the figure below.


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Front Principle When the forces have operated long enough, all the oil will be floating on top of the water, and the pressure gradient force will have ceased. Applying this illustration to the weather, the cold front has overtaken the warm front, and the warm, less dense, air will be overriding the cold air - the front has occluded. In the atmosphere wind does not blow from higher towards lower pressure but parallel to the isobars. Two air masses, one warm and the other cold can thus adjoin each other with the cold air as a wedge under the warm air, without the cold air being further pressed in under the warm air. A state of equilibrium will develop, where the slope of the frontal surface depends on the difference in temperature and the wind on both sides of the front.

The Slope of The Frontal Surface Frontal weather is intimately associated with the slope of the frontal surface, and in principle we can say, that the steeper the slope is, the more intense will be the vertical motion of the air. It therefore follows that a steep front (an active front) affects a very narrow area but produces a lot of weather, mostly cumulonimbus clouds. The shallow front (passive front) affects a larger area but the weather will be of the nimbostratus type with or without fog clouds. It must be pointed out that this simplified assumption implies that there is a convergence along the front creating an ascending motion along the front. The following approximate values apply to the slope of the fronts:

• The slope of the warm front is 1/ 100 - 1/ 150 • The slope of the cold front is 1/ 50 - 1/ 100.


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Pressure distribution around a front Now, let us first look at a homogeneous air mass without a front, e.g. a warm air mass. In the figure below to the left we can see such an air mass with isobars drawn in and with the lower pressure to the north. The air is moving from NW.

Now suppose we replace the air to the right of the wall A - B with a wedge of cold air, as if it were a front. Seen sideways we get the right figure with warm air to the left and the wedge of cold air to the right. Since the cold air has greater density than warm air, a hypothetical pillar of air, X, will be lighter than a pillar, Y, in the figure above. Let us now draw the figure again with the warm air mass seen from above, and replace the air to the right of the wall A - B with the wedge of cold air. We then get the following pressure picture. The line A - B in this case represents a front. The new pressure picture shows that a front is situated in a trough, and this is where we always find the fronts - in more or less well-developed troughs. When the front A - B moves towards an observer at point C, the air pressure will first be falling, until the front has arrived at C and then it rises again. Imagine a motion from C towards D in the figure above. The bending of the isobars also indicates that the wind will veer when the front passes the observer, from SW to NW in our example. On a recording barometer, a barograph, we will get a very clear indication of a frontal passage. We would see a continuous fall of pressure as the front approaches. When the front passes through, the fall of pressure will stop and then the pressure may start to rise again. The change of pressure during the last three hours, the pressure tendency, is among other things used to trace fronts and, on the weather chart, the tendency can be plotted as an isallobaric field. Did you remember that an isallobar is a line joining places that show the same value of barometric tendency at the same level and over the same period of time? Regions where the pressure is rising more than elsewhere are marked as ”rise regions” or isallobaric highs, and those with falling pressure are marked as ”fall regions” or isallobaric lows..


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Wind Around a Front How is the wind affected by the passage of a front? The pressure pictures shown so far have referred to ground level, the isobars. If we apply our knowledge of the gradient force and the Coriolis force we get the geostrophic wind, i.e. a wind which blows parallel to the equidistant isobars at a speed which is proportional to the distance between them. However, as we have noted the surface wind is affected by surface friction and backs somewhat towards the lower pressure. Just before the frontal passage the airflow will back a bit more until almost parallel to the front, resulting in a somewhat sharper turning of the wind towards the lower pressure than normal. As the front passes the observer, the wind veers (turns clockwise) at low level (in the Northern Hemisphere). The sudden changes of wind direction in the vicinity of the front are likely to give turbulence and wind shear. Let us first study a warm front.The figure illustrates that, at the front, there is a thermal gradient from the warm air mass to the cold air mass. As the front becomes steeper the thermal gradient intensifies and a strong thermal component will develop. This will provide a vector of flow parallel to the front with the cold air (low pressure) to the left. By adding this

vector to the lower wind we can see that, with increasing height, the wind will back.

Vs the geostrophic wind at low level (e.g. the surface wind) Vu the geostrophic wind at an upper level (e.g. at 2000 ft near the front) V T the thermal wind between the layers T temperature (isotherms represented by broken lines).

At the cold front we will see the same effect and if we carry out the same reasoning we will see that the wind at this front will back as we climb.


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Here we can state that if a warm front approaches an observer from west towards east, the upper level clouds will approach the observer from a northerly direction. When the cold front has passed and continues eastwards, the high level clouds associated with it will move from a southerly direction. These changes are what you could expect in an idealised situation. However you must keep in mind that, in the study of meteorology, no two situations are identical. The conditions will change from day to day and from locality to locality. Some of these changes may be masked by other effects and will not appear. We will now look at the conditions associated with fronts. The Warm Front When the warm air is overtaking the cold air, it will ride up and over the heavier cold air while advancing. The cold air forms a wedge in below the warm air. The slope of the frontal surface is gentle or 1/100 - 1/150 on an average. (The warm front can be compared to the bows of an icebreaker, which slides up on top of the ice and then presses it away. This results in a descending air motion in the cold air). The movement of the warm front can be estimated from the isobar spacing along the front. If we measure the spacing using the geostrophic wind scale along the warm front according to the figure, the movement of the front is about 70% of the “wind speed” read from the scale. This gives the vector velocity of the front in a direction of movement perpendicular to the front. This simple “rule of thumb” is only valid when the estimated movement is about 8KT or more. The average speed of the warm front is 20KT (10-30), but deviations may be great. That is to say, the front will pass about 24 hours after you have seen the first Ci clouds.


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Temperature at the Passage of the Warm Front As the warm front approaches the observer the temperature will begin to rise very slowly. At frontal passage the temperature may show a marked increase as the cold air is replaced by the warm air. Pressure change at the passage of a warm front The most common pressure pattern is that the pressure falls some hours before the front passes. This fall can occasionally be significant and rapid (about 8-12hPa/ 3hrs). After the front has passed the fall ceases or decreases. Sometimes the pressure rises an hour or so before it again starts to fall ahead of the cold front, which nearly always sweeps along behind the warm front. Wind change at a warm front passage Ahead of the warm front the wind will veer, eventually to blow almost parallel to the front. At the same time the wind speed increases, since the pressure gradient normally becomes steeper. After the frontal passage the wind backs. Note that there is a significant risk of wind shear when you descend through a warm front at low level. Humidity at the Warm Front. Because of the rain that will be falling the relative humidity in advance of the front will be quite high. As the front passes the warm moist air that now covers the observer will also have a high relative humidity so there is little change in the % saturation. However, at frontal passage both the dry bulb and dew point temperatures will increase. Clouds and weather at a warm front When the warm air rides upward over the cold air it cools adiabatically. The water vapour condenses into cloud droplets and, since the warm air is mostly stable stratified, the clouds become stratiform in appearance, Ci -Cs -As – Ns.

Above shows a simplified picture of a warm front. This is the idealised view of what it could look like out at sea or over level areas. Over broken terrain deviations may be great due to surface heating or cooling and mechanical effects. The first clouds we see when a warm front is approaching are cirrus clouds (Ci) often around FL 250-350. After that we find clouds at lower levels as the cloud veils grow more dense and turn into cirrostratus at Fl 200-250. The thickness of these layers varies from case to case, but it is generally a question of several thin layers of ice clouds. It is often possible to catch a glimpse of the ground through these clouds while flying on top.


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Closer to the front the clouds grow more compact and from FL 150-200 and downwards there are mostly compact clouds with only thin layers of clear sky in between. Depending on the distribution of temperature altostratus clouds consist of ice crystals, supercooled water droplets (0° -15°) or ordinary water drops. Precipitation starts to fall but it usually evaporates before it reaches the ground. In winter can it create hazardous icing conditions if its rain that falls into air with subfreezing temperatures below the warm front. As the precipitation reaches the ground the cloud will have developed into a nimbostratus and the cloud base has lowered to about FL 60- 80. The distance from the surface front is now about 150- 200 miles. The slow and smooth lifting at a warm front generally produces precipitation of moderate and constant intensity. Below the nimbostratus clouds there are stratocumulus and stratus clouds developing at all possible levels. The area of precipitation, about 200 miles wide, occasionally turns into frontal fog and drizzle 20 - 50 miles ahead of the frontal passage at the ground surface and Instrument Meteorological Conditions are very likely. Behind the warm front, in the warm sector, the sky will change radically and the upper-level clouds will vanish. What remain are the typical warm air mass clouds and weather at low level (stratus/stratocumulus etc.) and some inversion clouds at altitude, as e.g. sheets of altocumulus (Ac). Differences Dry warm air If the warm air is very dry, it must be lifted to high level before condensation occurs. In these cases only medium-high clouds (As, Ac) and high clouds (Ci, Cs) are generated, and any precipitation that may form will not reach the ground but appear as fall streaks (virga). Unstable warm air If the lapse rate in the ascending warm air modifies to an unstable condition, convective cells will form as the warm air is lifted along the frontal surface. The Cb clouds are not visible from below, but the intensity of the precipitation generally increases. At altitude, however, the Cb-cells are clearly visible. In this case we say that the Cb clouds are embedded in the rest of the mass of clouds and they lead to increased precipitation locally. Sometimes so-called rain bands occur, which are temporary increments of the intensity of the precipitation. In practice it appears, that there are usually two bands of more intense precipitation, one associated with the leading edge of the precipitation and another just ahead of the ground front.


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Flying conditions

in a warm front In a warm front extensive cloud systems exist both vertically and horizontally. Flying close to or through a warm front therefore always involves a lot of instrument flying. Turbulence is generally light, except for the very transition zone, where more severe turbulence may exist locally. If embedded Cb’s occur, ordinary Cb-weather will prevail with severe turbulence, possibly hail and thunder. Nimbostratus clouds may cause severe icing in a young front when there is a maximum amount of supercooled droplets, or in the winter over the sea, where the water content always is great. Exactly as in the case of Cb-clouds, the icing intensity will decrease when the cloud has drained itself of some of its humidity content. It is wise to avoid flying through frontal clouds within the temperature interval 0°- 5°C since in this layer there often exists a mixture of large supercooled drops and snow flakes. This leads to a high risk of severe icing if your luck is out. You should also keep in mind that, if the front is being driven against high ground, the additional orographic lifting will cause an increase in the liquid water content of the cloud and a consequent intensification of the icing. Visibility in the cloud masses varies, but in those parts of the cloud where precipitation occurs, flight visibility will be very limited, and if the drops are large, they will splash against the windscreen reducing visibility to nil. The size, number and speed of the precipitation droplets will also influence the visibility below clouds. In general we will have to allow for a flight visibility significantly worse than the values specified in METAR and MET REPORT. Stratus clouds often form in connection with precipitation. Fog, so-called frontal fog, also forms in situations where warm rain occurs. The uplands will nearly always be affected by low stratus associated with warm front rain.


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Remember that below the surface of the warm front there is a wedge of cold air. If the temperature of this air is below 0°, the raindrops will supercool and the risk for severe icing even outside the clouds will be great. If the raindrops have time to freeze, ice pellets appear on the ground, but there is still a great risk for severe icing in layers just above the ground. The Cold Front The cold front is generated when heavy cold air advances in under a warmer air mass forcing it to rise. The slope of the cold front varies a great deal, and we distinguish between passive or kata-fronts with a slope of 1/100 and active or ana-fronts with a slope of up to 1/50. If we measure the movement of the cold front (as in the case of the warm front earlier) the movement of the cold front is about 90% of the measured “geostrophic speed” along the front. That is, as a general rule, the cold front catches up the warm front. Temperature changes at the cold front As the cold front passes an observer, the cold air will advance and the temperature will drop. Pressure change at a cold front passage When a cold front is approaching, the air pressure starts to fall, or the tendency to fall increases. After the passage of the front the air pressure normally rises substantially as the cold, heavy air replaces the warm air. Wind change associated with a cold front passage Ahead of the cold front the wind veers until it is almost parallel to the front. Simultaneously the wind speed increases and closer to the front it will be gusty. After the passage of the front the wind backs and the speed gradually decreases. The advancing cold air may occasionally cause a stormy day or two before the wind abates. Humidity Because the cold air is normally maritime in nature the relative humidity remains high but is unlikely to be as high as that of the warm air it displaces. The dew point temperature will show a marked decrease. Clouds and weather in connection with a cold front The properties of a cold front are largely dependent on the speed of the front. The stability and humidity of the warm air are of decisive importance to the weather along the front. An unstable, stratified warm air mass and/or a rapidly advancing cold front leads to a steep and active cold front, while a stable, stratified warm mass together with a slow-moving cold front results in a shallow and passive cold front.


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Passive cold front, slow moving cold front, weak cold front or cold kata-front At a passive front the warm air is slowly lifted, and the cloud system looks like that of a warm front turned the wrong way. The slope of the cold front, however, is much steeper closest to the ground, which leads to the formation of convective cells at the leading edge of the front (see fig. below). Then follow the warm front clouds, turned the wrong way. Ns - As - Cs - Ci and in the area of precipitation stratus may form and, once in a while, fog - if the winds are light enough. If the warm air in front of the cold front is dry and stable stratified the cloud layers associated with the front might be reduced to dense layers of Sc and Ac. Note in the figure that the major part of the precipitation area will end up behind the front itself, but that the most intense precipitation will be found associated with the front. The precipitation area is 100 - 150 miles broad, and the front is moving with a speed of 10 - 15 KT.

Active Cold Front Or Cold Ana-Front When the speed of the cold front increases, the slope becomes steeper, and the warm air is simply pushed upwards. If the lifted warm air is humid the released latent heat will boost the activity. A complicated flow pattern is generated closest to the frontal surface and mature convective clouds appear in a narrow band, about 50 miles, in connection with the ground front. An active cold front is characterised by a band of heavy but brief rain showers followed by a rapid clearing up, later turning into a typical cold air mass. If the warm air is initially unstable you can expect heavy instability weather. Even an initially stable stratified warm air mass can generate convective clouds, and if the underlying surface reinforces the lifting of the air, as e.g. along a mountain range, some very nasty Cb clouds can develop. In the summer when we have an intense heating of the ground, or in the winter when cold arctic air is moving south, the most severe cold fronts are to be found in Northern Europe As you can see in the illustration below, there is not much weather right behind the cold front. The wind will be gusty and of varying direction. If the temperature contrasts across the front are great, a low-level jet may develop ahead of the active cold front.


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Low-level jet at cold fronts (LLJ) When the temperature contrast is great across the front, and the frontal surface is “vertical” at low level, this contrast will create a thermal wind in a strong wind band. The band blows parallel to the front and in towards the low pressure. The band is about 25 NM ahead of the cold front. Apart from the jet there are vertical updrafts in the warm air. These can be very strong up to about 7000 ft. On the cold-air side of the front we have downdrafts, and thus a small zone with very strong windshear will have developed along the front. The warm air, which gathers speed ahead of the cold front, slides up over the warm front according to the figure and a Warm Conveyor Belt (WCB) forms. It may hold rain showers and we get rain bands in the warm air mass ahead of the cold front and embedded Cb´s along the warm front. Upper Cold Front The cold front may sometimes start to incline forwards at upper levels (above 10 000 ft), and relatively cold and dry air may move in over the warm sector. Cold air above warm air is tantamount to unstable conditions, and convective cells will start to ascend from the warm air. The convection is strengthened even more along the warm front due to general lifting along the frontal surface leading to intensified precipitation along the warm front. The frontal clouds will drift away from the original cold front, and we talk about an Upper Cold Front (UCF). The cloud tops will descend behind the upper front, and only low clouds will persist with Cu /Cb embedded among St and Sc. Accordingly there may be several bands of precipitation inside the warm sector and no or only occasional showers along the cold front at the ground. The distance between the ground front and UCF can be close on 130 miles. The UCF is often marked with a broken or hollow cold front line on the analysed surface weather charts.


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Dry warm air ahead of the cold front If the warm air is dry, the cold front can pass without any precipitation falling and occasionally with almost no clouds. The passage of the front is then only noticed as a wind shift and generally an improved visibility as the colder and mostly cleaner air mass is moving in over the area. Flying conditions in a cold front A cold front can be associated with anything from thunder to altocumulus with small falling streaks. Every flight close to or through a cold front must therefore be carefully planned. When you intend to land at the same time as a cold front passes, you must be particularly careful when studying that front. How about Windshear? Turbulence? Icing? Electrical phenomena? Runway conditions versus a gusty crosswind? Other frontal weather? etc. If you operate light aircraft the best thing to do might be to land and wait while the front passes over. The Occluded Front When a cold front catches up to and overtakes a warm front, the frontal boundary is called an occluded front. There are now three adjacent air masses, cold air ahead of the warm front, warm air in the warm sector (lifted off the surface) and cold air behind the cold front. As the cold front overtakes the warm front, the lighter warm air is forced to ascend and the subsequent course of events depends on whether the advancing cold air is warmer or colder than the air ahead of the warm front. The warm front occlusion is generated when the coldest air lies ahead of the fronts, and the cold front occlusion when the coldest air is behind the cold front. The properties of the occlusion vary from time to time and in effect combine the properties of the original fronts. Each occluded front must be studied carefully. Its properties depend on • the humidity content of the air masses • their stability and • how old the occlusion is. In association with the occlusion process upper fronts also form. These indicate the old boundaries between warm and cold air. At many weather service stations these upper fronts are analysed and marked with a broken red line or a hollow warm front indication to signify an upper warm front, and a broken blue line or a hollow cold front indication signifying an upper cold front.


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On a surface weather chart, an occluded frontal system show three typical fronts joining together in the “occlusion point” or triple point. Those are the occlusion front, the cold front and the warm front. After the occluding process has persisted for a while, the contrasting air masses will become equalised, and the weather system is beginning to dissolve. An old occlusion therefore seldom constitutes any major problem as to flying weather at altitude, but stratus clouds may still exist at low level. Back-bent Occlusions Sometimes the low pressure centre remains close to the occlusion point and the occluded front twists around the low. This is known as a back-bent occlusion and it can appear as a new warm or cold front sweeping down behind the occluded low.


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Families of Extra-Tropical Cyclones (ATP Level only) When a disturbance once has developed along the Polar front, it mostly leads to the forming of a new one, and then yet another etc, etc. Finally we have a whole series (or family) of frontal depressions (Extra-tropical Cyclones) at various stages of development, and in between there are small ridges of high pressure.

Family of frontal depressions. Bad weather is seldom a solitary phenomenon but is mostly part of a series creating alternating weather. A Cloud amount at height increases in front of the warm front. The stability in the cold air

below the approaching warm front increases and the convective clouds turn into stratocumulus. At height will the tropopause levels increase.

B The extensive warm front rain creates cloud formation at lower levels and stratus and fog can occur.

C Inside the warm sector, the humidity and stability of the air and the temperature contrast in respect to the surface determine the weather conditions. But usually in this area stratus, advection fog and drizzle are common.

D The cold front is associated with showery precipitation now and then with hail and thunderstorms.

E Just behind the cold front is the convection reduced due to sinking motions and higher cloud levels reducing the insolation. The low level weather is often clear but winds might be gusty.

F Further away from the front is convection blooming up and showers develop. This area shows the typical cold air mass signs.

G The travelling ridge of high pressure in between the frontal depressions stabilises the weather pattern, convection turns into scattered stratocumulus below the subsidence inversion. Winds calm down and radiation fog might develop over land during the nights.

Cold air, however, is being forced towards the Equator, so the track of the subsequent frontal waves is moving further south all the time.


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Secondary Fronts A front is the boundary between two different air masses. Sometimes differences in temperature and humidity arise within the same air mass, and we then talk about a secondary front. Secondary fronts mostly occur in the cold air behind a low pressure, when the air, which is moving south, passes over different underlying surfaces, land or sea. During its motion over water the air picks up humidity, and becomes somewhat unstable as the temperature at low level rises. If air moving over land encounters airflow from the sea, a front is generated and the warmer air rises. But the process is limited both horizontally and vertically and mostly affects only a narrow zone. However, the weather can be intense with steady precipitation and low cloud bases. Trough We sometimes find troughs, in addition to the frontal troughs, around depressions. In a trough the wind converges at low level. This leads to the strengthening of an initially ascending air motion. If the humidity is sufficiently high, the lifted air condenses, forming convective clouds with rain-showers. Moreover, if the air mass is initially unstable, bands of very intense Cb´s form along the trough. Most troughs are found in the cold air behind the frontal low. The cold air from the north is advected down over a warmer, underlying surface and is destabilised before the high pressure ridge between the lows begins to affect the weather. Flying weather in a trough line Violent Cb´s should always be regarded with great respect, and the Cb clouds are usually exceptionally intense along trough lines resulting in thunder, hail and severe turbulence. The figure illustrates how a trough line often sweeps down into the cold air behind a cold front or an occluded front. A hooked line marks the trough line. Squall Lines Ahead of convective cells of cumulonimbus, there is an outflow of cold air (downdraft or gust front). If the warm air which is forced aloft is moist and unstable or in a state of conditional instability, very intense Cb cells rapidly form which will prevail over the mother cloud. The Cb tops of the recently formed cells are initially much lower than the original clouds, but the weather becomes more intense and downdrafts of the downburst type occur. Over larger continents tornadoes may form. After a short while (15 - 30 minutes) the proportions of the new Cb clouds may become significant. Ahead of a cold front or a trough line the outflow of cold air can be fairly uniform and at about the same distance from the mother clouds a line of new CB clouds forms - a squall line. Mountain Effects On Fronts When a warm front passes a mountain range, the air, which is lifted over the mountain, will strengthen the formation of clouds and the precipitation on the windward side. Moreover, the retreating cold air may be trapped between the mountain and the frontal surface. The result is that the warm front sometimes seems to halt in front of the mountain, (“stau” in the Alps). On the leeward side the air is being adiabatically heated as it subsides, which brings about decreasing precipitation and occasionally cloud dispersal - ”Foehn-opening”.


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Not until well beyond the mountain does the frontal surface resume its original appearance with clouds and rain similar to the conditions prevailing before the passage of the mountain. To the observer or meteorologist, it seems that the front has come to halt then dissolved. It then suddenly reappears out of the blue a distance beyond the mountain. The many forecasting problems mostly show as vaguely formulated TAF’s (Terminal aerodrome forecasts) in the area. Note that flying weather deteriorates radically on the windward side of the mountain, and you had better prepare for possible Cb-weather if you intend to fly through the front. If the air on the leeward side is cold and heavy it may persist, and the warm air glides away as an upper air mass and an upper front. Once again the meteorologist will meet problems, since the change of air mass will not appear in the observations from the ground. A cold front lifted over a mountain is always strengthened In winter, the air behind the mountain may be colder at low level than the approaching cold air mass. The cold front then passes the top of the mountain and continues as an upper cold front (cf. with the warm front). If the coldest air is found behind the cold front, the extra speed given to the cold air downhill may strengthen the cold front some distance behind the mountain, where the ”warmer” air is pushed up, forming a Cb cloud. We mentioned earlier that a frontal passage does not always imply an expected temperature change. Here below are three such cases: Cold Front Leading To an Increase Of Temperature There is often a strong wind and heavy turbulence behind a cold front. If there is a ground inversion with low temperatures ahead of the front, it will disintegrate, and the passage of the front will marked by an increase of temperature. Warm front passage that does not lead to an increase of temperature In wintertime with snow and well-developed ground inversions, this inversion sometimes persists even after the passage of the warm front. On the surface weather map it is not possible to notice any change of temperature or dew point after the frontal passage. Another example occurs on warm and sunny summer-days, when the growing cloudiness ahead of the warm front prevents insolation and thus lowers daily temperatures. The cooling, due to precipitation, will further reduce the temperature.


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COLD FRONT EFFECTS

PROPERTY AHEAD IN THE COLD SECTOR

AT THE PASSAGE OF THE FRONT

BEHIND IN THE WARM SECTOR

Wind Veers and strengthens

Backs and weakens Direction steadies

Dew Point Temperature

Falls steadily Rises Steadies

Pressure Tendency Falls steadily Remains steady Small change or falls slowly

Temperature Slowly rises Rises Small change Visibility Good except in rain

or snow Often mist or fog, poor

Poor to moderate, mist or fog

Cloud types Ci, Cs, As, Ns, Fra, St, Fra Cu, below As & Ns

Low Ns and Fra St St or Sc or no cloud Maybe Ci

Weather Continuous rain or snow

Precipitation nearly stops or does stop

Intermittent drizzle or none

WARM FRONT EFFECTS

PROPERTY AHEAD IN THE COLD SECTOR

AT THE PASSAGE OF THE FRONT

BEHIND IN THE WARM SECTOR

Wind Sudden Veer and strengthens becoming equally

Suddenly Backs and perhaps a squall

Veer after squall or backs later in more squalls

Dew Point Temperature

Little change Suddenly falls Little change

Pressure Tendency Fallls Rises suddenly Slow continual rise Temperature Steady but drops in

pre frontal rain Rises suddenly Steadies except in

showers Visibility Steady but drops in

pre frontal rain Falls suddenly Very good except in

showers Cloud types St, or Sc Ac As then

Cb Cb with Fra St and Fra Cu or low As

Rapidly clears some As Ac, then Cu or Cb

Weather Drizzle Heavy rain, snow, hail, maybe thunder

Heavy rain or snow then fine with showers


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Pressure Systems On the weather charts there are different types of pressure systems with different names depending on how the systems are generated. As a well-educated pilot you should know how these differences form and what sort of weather is typical for the various pressure systems. By the end of this chapter you shall be able to Describe the formation and properties of:

• Thermal or cold and dynamic or warm anticyclones, • Ridges or wedges. • Non-frontal depressions • Tropical revolving storms • Easterly waves • Tornados or Whirlwinds • Frontal or travelling depressions

The air pressure may vary considerably between different places on earth. These pressure differences are of decisive importance to the Earth´s weather and winds. On the charts of the weather service we find the pressure pattern, delineated by the isobars, enclosing areas of different pressure;

• High pressures and ridges, • Low pressures and troughs.

Some very old ship’s barometers, and most barometers that are displayed in homes, declare that low pressure means rain and storm and high pressure calm and sunny weather. These simple declarations were the results of many years experience compressed into one measurement and a simple forecast. In this chapter we present the various pressure systems. The old sailors were not so far wrong in their assumptions, even if conditions are somewhat more complicated than they thought.


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Anticyclone or High, and Ridge or Wedge A high pressure system is an area enclosed by isobars the values of which decrease with distance from the centre. Nature of a High In a high the isobars are mostly widely spaced resulting in light winds but in areas where a high is brought close to a low the pressure gradient can become steep and moderate to strong winds (and also dust storms over arid areas) can be experienced.

When we study the vertical airflow in a high pressure cell, we see mass convergence at height and divergence at low levels creating a descending air movement or subsidence within the core of the anticyclone with an outflow at low level. The subsidence is checked at some height above the ground, due to the thermal mixing in the surface layer and a subsidence inversion is formed. The height of the subsidence inversion depends on the intensity of the anticyclone, the thermal mixing and on the distance from the core. Values from 2000 to 5000 feet are not unusual for cold anticyclones while warm anticyclones can show values up to FL 100. Above the boundary layer, the wind blows in a right-hand circuit parallel to the isobars. In the boundary layer, friction slows the wind down and it will blow at an angle out from the centre. The outflow at the bottom leads to a sinking motion of air through the high, and the air is compressed and adiabatically heated. The result can be seen as a subsidence inversion, within which the temperature rises significantly and the humidity decreases. Note, in the Southern Hemisphere the rotation is reversed. The air above the inversion is dry, while the air below may or may not be humid depending on the circumstances that prevail. Air pollutions collect below the inversion, and this leads to an impairment of visibility in the stable air in the lowest levels. If the airflow is such that the inversion persists for a day or so, clouds will probably form in the inversion. At high latitudes, the increased loss of terrestrial radiation due to the drying at height creates nocturnal inversions at the ground level and large areas with Sc and St clouds may form in maritime air masses. In the winter these clouds may persist for several days, making VFR-flights impossible and cause poor weather for IFR-landings.


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When the humidity is high and the lower levels are cold, there is a great risk of fog forming below the subsidence inversion. In the summer or at lower latitudes the clouds often dissolve during the day and reform at night. They are mostly Sc clouds. If the air below the subsidence inversion is unstable or conditionally unstable, Cu may form below the inversion in the day. In continental air masses the humidity content is lower, but in spite of that, visibility is mostly limited below the inversion. If the air passes major lakes or a sea, it will rapidly absorbs humidity, which leads to the formation of clouds. In the same way, the maritime air mass will dry out with an extensive passage over a major land surface. Flying weather is generally good above the subsidence inversion, that is, it is cloudless and good visibility values. High Pressure Types There are two main high pressure types, depending on whether they consist of warm or cold air. We assume, that the distribution of temperature is symmetric around the centre, otherwise the system will slope and we won´t get a true presentation of the pressure. Subtropical Highs (Warm Anticyclones) These highs are caused by air from the equatorial regions streaming northwards (southwards) at altitudes around the Equatorial tropopause. They are deflected to the right (left), which generates the subtropical jet and an accumulation of air around the 30º latitude north (south). At low level the air pressure increases and there is a return flow out from the system. Within the subtropical highs there is subsidence from aloft and the centres consist of warm air. The subsidence inversion in these cells is sometimes called Trade inversion. These anticyclones are often stationary or very slow moving (more or less a seasonal movement) and are therefore referred to as permanent highs. Towards the Polar side of the centre itself, the high pressure is clearly visible on the 500 hPa charts (FL 180) in the summer. In the winter, however, the high may be difficult to identify above 700 hPa (FL 100) at the same latitude. The air below the subsidence inversion is humid and unstable, above it is dry and stable. The weather is dominated by a Cu-sky below the inversion. The height to the inversion varies within the high pressure cell. The highest values are found in the western parts nearest the Equator (5000 – 7000 Ft) and the lowest in the north-eastern part ( 1500 – 2000 Ft). This is the reason why Tropical showers are more likely to develop in the western part of an ocean than in the eastern. On its way to the north the low-level air normally picks up large amounts of humidity. At the same time, as the sea temperature decreases, the air is cooled from below and, in the winter, this frequently leads to vast areas of low clouds, drizzle and fog when the air reaches the southern African coastline. In the summer the anticyclone occasionally is intensified over the North Atlantic. This causes lows and their associated rain areas to move in a wide arc north of Scandinavia, forming a blockage (”a blocking high”) with dry and sunny weather over western Europe.


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Continental Highs (Cold Anticyclones) These consist of polar air and form over the cold continents in the winter. At low level they consist of chilled air and seldom reach higher than 700 hPa (FL 100), but their horizontal extension may be considerable. The thermal highs are not as stable as the dynamic ones, and travelling depressions may temporarily break them down. The Siberian and the Canadian highs consist of, and are the source regions of continental polar air. In midwinter they also constitute the source region of arctic air from within the arctic and antarctic permanent cold anticyclones. On the whole, weather will be as in the description of an anticyclone at the beginning of this chapter. If the pressure system spreads over a coastal area, e.g. along the coasts of the North Sea or the Baltic in the winter before any appreciable icing has started, there will be convection and snow showers over the open water surface and mostly fog and mist below the inversion inland. If the air is dry and there are no advection from open water weather can be cold, bright and cloudless. In clear and extremely cold areas, ice fog or diamond dust may form. Even aircraft can generate ”fog” along the runway by mixing the extremely cold surface air with somewhat warmer air a few feet above the ground. High Pressures And High Pressure Ridges (Or Wedges) In Series Of Travelling Depressions The third type of high pressure forms between the lows of a family of depressions. These ridges, or sometimes temporary highs, form as cold air sweeps down behind a frontal low. Hence, this type of high is thermal, and as a consequence it is not visible on an upper air chart. The high pressure ridges follow the lows in their movements and constitute a break in the storms associated with the frontal systems of the lows. The ridge can be subdivided into three weather zones; ahead of the axis of the ridge (i.e. just behind the cold front), along the axis of the ridge and behind the ridge (i.e. in front of the next warm front). Generally speaking it can be stated, that there is a high risk of showers, often troughs with low pressure and line squall showers/thunderstorms well ahead of the ridge axis. CB turns to Cumulus and later Stratocumulus closer to the ridge axis and it might clear up. In winter radiation from the ground is great, and nocturnal radiation fog is likely to form if the wind is light, or St or Sc if the wind is stronger at the border of the ridge/cold high. When the ridge has passed the air will be humidified in the prevailing SW-wind, which again leads to increased cloudiness with cumulus and stratocumulus at lower levels while the frontal cloud deck thickens at height.


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When the last depression in a series has passed, a major outbreak of cold air generally occurs, which creates a cold high. This high is north of the subtropical high pressure cell in the polar air (if we talk about the polar front, which is the most common position). In this case, the subtropical high pressure cell will weaken and the thermal high will intensify and replace the subtropical high. After a while, however, the cold air will be heated and transformed into tropical air, and the air masses and pressure distribution will have returned to their original positions. We call this regeneration. Low, Cyclone or Depression and Trough In a low the isobars are normally closely spaced, resulting in windy weather except in the centre where the wind is calm. The air is sucked in at low level (convergence) and is then forced aloft and cools adiabatically. If the air is initially humid, this general lifting leads to condensation and the forming of clouds

Above the boundary layer, the wind follows a left-hand circuit parallel to the isobars. Friction acts as a brake on the wind in the boundary layer, and the wind blows at an angle towards the low pressure centre. The result is a general lifting of the air within the low pressure area. In a low pressure area convective movements, if any, will be strengthened, and Cb-clouds are likely to form if the air is or becomes unstable. However, even if the air is stable, clouds will form if the air is humid enough, and in this case there will be extensive stratiform cloud layers. Visibility at low level is generally better than in an anticyclone, due to a stronger mixing of the air.


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Low Pressure Types As in the case of the high pressures there are two basic types of lows, warm and cold.

A. Dynamic = cold low The low is deepening at altitude and the winds are increasing B. Thermal = warm low The low is weakening aloft and turns into a high pressure E.g. the Asiatic warm low

The adiabatic cooling (due to the expansion of the air) generally leads to extensive cloudiness in the whole low pressure area. At our latitudes there is a transport of unstable ”cold air” in the northern and western parts of the low, while there is an airflow of more stable ”warm air” in the southern and eastern parts. Showers are therefore more frequent in the north-western parts of the low. Apart from the showers, visibility is often good in the low. If the lifted air is sufficiently humid, extended As / Ac layers with embedded areas of light rain may form. The Origin Of Low Pressures And Weather From the aviation point of view there is a better grouping than dynamic and thermal, and we usually consider the following different types and sub-divisions: Non-frontal depressions:

• Orographic lows • Thermal lows • Summer lows over land or • Monsoon low • Equatorial low or trough • Instability lows • Winter lows over sea • Mediterranean low • Polar low • Baltic Sea low • Cold air pool • Tropical revolving storm • Easterly waves • Whirlwind or Tornado

Frontal depressions Orographic or lee-side lows or troughs When a current of air flows towards a mountain range, especially one with north-south orientation, the barrier will force the air to compress on the windward side and over the mountain and the air will thereafter stretch on the leeward side of the mountain. From the air circulation point of view, there will be a tendency for anticyclonic curvature over the mountain with closely spaced pressure surfaces, and on the lee-side there will be a clearly visible cyclonic curvature.


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This leads to falling air pressure, and a ‘low’ forms on the leeward side of the mountain. This is known as a lee-depression or a lee-trough. The lee-trough is usually stationary as long as the air stream remains the same and no deepening low forms. These figures show how orographic lows may form.

The lee-low causes the pressure surfaces to slope down towards the mountain, and over the mountain the pressure surfaces are closely packed. This may be a flight safety risk in itself, if the margin between the flight level and the height of the obstacle is small. Behind the mountain, foehn winds prevail, and the weather is generally fine (or at least better than in the surroundings), but humid air may be sucked into a lee trough giving clouds and sometimes also precipitation. When a cold front encloses warmer air on the leeward side of the mountain, this generally leads to a rapid development of the system. The low will deepen and intense cumulonimbus clouds will form.

When the cold air sweeps round the sides of the mountain and across it, the warm air on the leeward side will act as a warm sector and a wave forms on the front. This wave normally develops rapidly. This leads to an occlusion-like process, and the storm moves away from the mountain.


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Thermal depressions Thermal depressions form over warm surfaces, which in most cases consist of a continent or some other major land area. The heated air rises through convection and turbulence, and after a while a high pressure aloft is formed. This causes an outflow of air and the air pressure at the ground decreases, similar to the sea breeze system but on a larger scale.

There is an influx of air towards the ground low and an ascending motion is generated, strengthening the convective clouds in the area, if any. Thermal depressions are often sub-divided into

• Monsoon low • Summer lows over land • Equatorial low or trough

The most predominant thermal depressions are the monsoon low pressures in Asia, the Equatorial low pressure belt and the summer lows in south-western USA and north-east Africa. However, lesser heat lows are very common on the weather charts in the summer. The smaller cyclones are shallow and don´t affect weather to any greater extent. We will deal with the weather in the Equatorial trough and the monsoons in a separate chapter and here we will concentrate on summer lows. If the air is dry, the heat lows mostly bring good flying weather with slight cloudiness and moderate to good visibility. When the air is humid, however, convective clouds of the CB-type are likely to form, and some heat thunderstorms or squalls will also appear. This is a rather common feature in France and on the Iberian Peninsula. Thermal lows generated in these areas may drift towards north-western Europe and Scandinavia. Remember that extremely warm weather affects flying in many ways. The high temperatures result in lower density and therefore reduced aircraft performance. From the physiological point of view, the warm and humid weather may in the long run cause heat stroke, and prolonged exposure to this type of weather can adversely affect the pilot’s performance. Instability lows If large scale “organised” convection takes place, resulting in convective clouds and precipitation, within an area where there already initially is a lee-low, a development may take place that looks similar to a thermal low. This is an instability low.


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The same process that created the thermal low, i.e. heating, also influences the instability low. The difference is that in this case a significant part of the energy is derived from the released latent heat of the condensation process. According to the hydrostatic equation, heating causes the distance between two pressure surfaces to increase. As a result of this a high pressure is generated aloft resulting in an outflow of air and falling pressure at the ground. If there already exists a divergence at height, the effect will be strengthened and a rapid pressure fall may occur at the surface level. This generates a spiral flow in towards the centre. Instability lows can be very intense, particularly in the Tropics (Tropical Revolving Storms). In mid-latitudes the humidity content is less (the amount of humidity in the air depends on temperature, as we know) and the lows are thus less intense. Polar lows Instability lows often form when cold polar or arctic air moves south over a gradually warmer sea or major lakes. Examples of this can be found in the northern parts of the North Sea, North Atlantic and North Pacific in the period from November to March. In these cases the air will transform due to an intense heating and vapour increase in the lower levels resulting in intense convection. A common phenomenon occurs when arctic air moves down over the Norwegian Sea with a fresh N to NE wind. Behind Lofoten and to the lee of Vestlandet, lee-lows form. These instability vortices are also called Polar Lows. They are small (about 300 nm wide) but violent lows, formed as a Revolving Storm with a typical Eye or sometimes as a large ”comma”, with long bands or clusters of convective clouds, heavy showers/squalls and a strong gusty wind. Occasionally a vortex may slip past Norway and reach the British Isles or the Skagerak area and further down to southern Scandinavia/ North Germany as an intense snowstorm. Between the two highs there is a tendency to a cyclonic airflow and the formation of lee-lows off the south-eastern coast of Norway. When the cold air reaches the warmer water, small intense instability lows develop, Polar lows. A similar type of instability low forms in the winter on the Bay of Genoa. They are generated when the cold Mistral wind sweeps down over the warm Mediterranean. Cells of cold air aloft, cold pools When extratropical cyclones are discussed we will consider the general theories about the long waves encircling our globe, separating the cold polar from the warm tropical air. These waves are usually zonal, that is moving from west to east along a latitude line, but sometimes cold air outbreaks, cut-off from the main stream, generate a pool of cold air at height in a position far south of the normal Polar front. This cold pool can remain for several days constituting a potential area of instability at height. In the summer warm lows form over the continents, and sometimes these may develop into instability lows. This happens when cold air is carried in over the low (by the upper airflow) or when a cold pool already exists at height. In these conditions the atmosphere becomes unstable, and a major area of thunderstorms may develop. These thunderstorms interfere with aviation in the area, since it may be difficult to fly round them.


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Tropical Revolving Storms (TRS) or Tropical Cyclones The tropical revolving storm is the most devastating storm on earth. The hurricane Gilbert e.g. caused damages of several thousand million dollars, at least 200 people were killed and thousands had their homes destroyed when the storm struck Jamaica and north-eastern Mexico in the middle of September, 1988. The intensity of the vortex is indeed less than that of a tornado, but the size is much larger. Nature Of The Tropical Revolving Storm The system as such is a cyclonic circulation that develops from a cluster of convective clouds that intensifies, first to a Tropical disturbance (wind speed less than 20 KT), then into a Tropical Depression (wind speed 20 – 34 KT), a Tropical Storm (35 – 64 KT) and then finally into a Tropical Revolving Storm (>64 KT) or Hurricane or Typhoon or Tropical cyclone depending on the geographical area concerned. A TRS can only form where certain definite conditions are fulfilled. Among these are:

• Warm and humid surface, water temperature of at least 27° C. (24°C for survival.)

• Uniform Trade winds at all levels, otherwise the Cb-clouds will be scattered. • Sufficiently unstable atmosphere at low level for the convective clouds to penetrate the Trade wind inversion. (This is possible for air moving east with the trade winds and being thoroughly humidified at low level).

• Divergence in the upper part of the troposphere poleward of the area where large scale convection develops, e.g. a branch of the subtropical jet interacts with the area.

Influence of the Coriolis force since vorticity is an essential part of the circulation, means that tropical cyclones do not occur at the Equator or within 5° latitude. The remains of a TRS can reach the Equator area e.g. in the Borneo region. Tropical cyclones do not form in areas (during periods) where the monsoon flow is strong. It is quite clear from what has been said above that the tropical cyclones can´t form over land, because it is too dry. Neither do any tropical cyclones form over the south Atlantic, because the sea currents keep the water temperature down(require at least 26°c) and the ITCZ is usually north of the Equator. A third restricting factor is the Trade wind inversion, which is lowest over the eastern parts of the oceans and highest in the west. In the eastern parts convection is checked almost at once, while in the western parts the rate of ascent can be considerable, making penetration of the trade wind inversion possible. This is why we find fewer hurricanes in the eastern parts of the oceans, while they are fairly frequent in the west.

The name and time of occurrence varies over the earth;


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• In the Atlantic and eastern Pacific N.H. they are called hurricanes, occur mainly in the period July to November, frequency about 10 times a year.

• In the Indian Ocean they call them Cyclones, occur April – May and Oct.- Nov. Occasionally (less than 5 times a year).

• Off the coast of East Africa they also use Cyclone, Nov. – April, occasionally.

• In the western Pacific N.H. and South China Sea, the name is Typhoon, June – November, this is the peak TRS area with a frequency of about 20 systems a year and a small TRS possibility the whole year around.

• In the western Pacific S.H. they are known as Cyclones, Nov. - April.

Most cyclones form between 8 – 15° latitude north and south of the Equator. They are then caught by the prevailing easterly flow in the tropics and carried away westwards or north-westwards according to the arrows on the map at a speed of about 10 to 20 KT. When the TRS passes the 25-30° latitude they ”re-curve” pole-wards in a north, later north-east direction, (N.H.), and in a south easterly direction in the southern hemisphere. When reaching a continent or cooler water surface the radius increases, the wind speed decreases and the system turns into a ”normal” extratropical cyclone. The re-curved TRS moving into the west winds poleward of 30° latitude usually accelerate and speeds faster than 30 kts are common. If the storm hits land before the whirl has exhausted itself, not only the hurricane-winds but also a flooding will cause destruction. A pillar of water, the Storm Surge or ocean swell, is created by the strong circulation in the system and the pressure from the on-shore wind will do the rest. The flooding may rise to 15 to 30 feet or more above the normal water level, and in regions near the coast the most common cause of death is drowning, while most people survive the gusts of wind. Ocean swells travels outwards from the system at an average speed of about 1000 miles a day which is several times faster than the TRS movement. Tropical Revolving Storm From The Pilot’s View The typical characteristics of a fully developed TRS are circular isobars, strong gusty winds, spiral cloud pattern and a typical eye in the centre. From a pilot’s point of view all TRS should be avoided, because there is always a potential risk of severe turbulence in and around the CB-spiral bands and especially in the border around the eye. At low levels devastating winds and turbulence close the airports. The change to lighter winds occurs at heights above 30 000 feet or so in the outflow region from the TRS. Due to the cyclonic circulation in a TRS, a flight with the eye on your left side will result in a tailwind component in the Northern Hemisphere. This also means that the most intense parts of a TRS is found in the right front quarter of the storm where the storm movement and winds interact while the weakest portion is in the left rear quarter. In the tropical regions there are often extensive areas with Ci-clouds. These are no real threat to flight but it is vital, when operating in these areas, to distinguish between these clouds and the spiral band CB.


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In a cumulonimbus cloud, containing a lot of water vapour, an enormous amount of energy is released during the condensation process. The released heat reduces the distance between the isobars, resulting in a thermal low with steeply inclined pressure surfaces. At this stage there is an inflow of air below about 7000 feet and an outflow of air in the upper troposphere from about 30 000 to about 50 000 feet. In short, a cyclonic rotation is generated with an accelerating helix, which pulls even more humidity from the sea etc. A number of CB-cells form, first in the shape of a comma, later as a circular helix with a distinct eye in the middle containing a subsiding return flow (compare with the spin-dryer in the laundry).

The eye (1) about 10 to 20 NM in diameter, The eye wall (2) of embedded CB clouds adjacent to the eye with tops at 40 - 50 000 feet MSL or more, and the convergence lines (5 to 30 NM wide) with showers spiraling inwards (50 to 200 NM apart) towards the centre of the storm, also known as rainbands (3). The areas with the strongest winds are found 50 to 100 NM outside the eye. The wind speed also depends on what side of the hurricane’s direction of movement you are. The highest speeds are on the side coinciding with the direction of movement and this is the area that sailors try to avoid. This is the right side of a N.H. TRS. Speeds in excess of 150 KT are not uncommon, while the system itself moves at about 10 KT. Wind weakens at height and the cyclonic circulation turns into an anticyclonic, this happens above about 40 000 MSL. The weather intensity is also increased in certain areas of the TRS. This is especially true in the leading inside sector, north-east quadrant in a TRS moving westwards on the northern hemisphere, since this is the quadrant where a large amount of humid air is drawn into the system. The system’s direction of movement is determined from an average wind between the Trade wind at low level and the upper wind at the top of the system. Two processes sometimes result in the storm intensifying into a super hurricane. This happens when there is a ”pool” of cold air aloft, leading to increased instability and thus stronger convection, (besides, the cold air loses potential energy as it subsides to the south exactly as at a front). The other process is generated when there is a jet stream aloft, which intensifies the outflow from the upper


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anticyclone in the TRS. Increased outflux reduces the pressure at lower levels, and suction into the cloud system is strengthened. In the central equatorial Pacific the trade wind systems of the two hemispheres converge in the Equatorial Trough and wave disturbances may be generated if the equatorial trough is sufficiently removed from the equator (usually 5°) to provide a small Coriolis force. These disturbances quite often become unstable forming a cyclonic vortex as they travel westwards towards the Philippines, but the winds do not necessarily attain hurricane strength. But there is yet another explanation of how tropical cyclones originate, and this is probably the most common reason for tropical cyclones in the Atlantic. Tropical Revolving Storm Dissipation When the tropical cyclone moves in over a cooler surface (less than 24°C) the winds abate and the eye of the cyclone is filled up with clouds before the whole system dissolves. Occasionally the storm comes in contact with the polar front and an extra tropical cyclone forms on the polar front. Cyclones of this type are recognised by their high wind speeds (often > 32 m/s) and elliptical form with clouds in a helical pattern. When a TRS develop it is given a name, special for that region, and the Weather Service responsible for the TRS tracking issues special bulletins and significant meteorological warnings, SIGMET. In USA a hurricane watch is issued when a TRS is expected in the area within a day or more, and a hurricane warning is issued when the arrival is expected within 24 hours. The following is an example of a Hurricane warning. NNNN ZCZC 111 Xxxxxx xxxx 302200 HURRICANE DAVID MARINE ADVISORY NUMBER 19 NATIONAL WEATHER SERVICE MIAMI FL 22002 AUGUST 30 1979 HURRICANE WARNINGS IN EFFECT FOR PUERTO RICO, VIRGIN ISLANDS, SOUTH COAST OF DOMINICAN REPUBLIC AND SOUTHWEST PENINSULA OF HAITI. HURRICANE CENTER LOCATED 16.8 NORTH 67.1 WEST AT 30 / 22002. POSITION EXCELLENT BASED ON NOAA RECONNAISSANCE, TIME OF FIX 17382, LAND BASED RADAR AND SATELLITE. PRESENT MOVEMENT WEST NORTHWEST OR 285 DEGREES AT 11 KT. DIAMETER OF EYE 20 NM. MAXIMUM SUSTAINED WINDS 130 KT GUSTS TO 150 KT……….. FORECAST VALID 31 / 06002 17.2 N 68.5 W…….


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Tornado (Waterspout Over a Sea Surface) The most intense weather is associated with the tornado, which is a violently rotating wind associated with an extremely intense cumulonimbus cloud often known as a supercell CB. A Tornado is visualised as a trunk hanging down under the CB. If this trunk isn’t reaching the ground it’s known as a funnel cloud. The tornado is principally a US phenomenon that arises when supercell CB’s form in areas with extreme instability. This can happen at all mid-latitudes but it is a far more common feature over the central and Southeast USA in spring and early summer when warm and humid Mexican Gulf air interacts with the cold air from north-western USA – Canada. A similar phenomenon occurs at other places in the world and is then called ”violent whirlwind” or ”small whirlwind”. The difference is in size and wind speed. The diameter of the tornado may be about 500 m, the violent whirlwind forms below less intense thunderstorms and is usually between 50 and 250 m in diameter. The small whirlwind, which is also mentioned in this context, is not associated with any Cb-cloud but is a vortex generated over intensely heated ground surfaces or in areas with strong wind shear. The small whirlwind reaches a maximum diameter of some 10m or so and the wind speed increases to about 20KT. Any loose dust or rubbish will be sucked up, and we talk about Dust Devils. In arid areas heating may be considerable and so also the damage caused by the vortex. Dust devils in these areas reach a diameter of about 30m and a height of about 300 feet, wind speed may increase to 50 KT and the life time can increase from the normal few minutes to hours. The Origin Of The Tornado Since the tornado is so short-lived and erratic it is yet not completely researched, and forecasting can be a problem. The rotation, inwards and up towards the cloud base, is probably started by strong crosswinds forcing the ascending warm air into a helical path. Initially the rotation is slow but increases gradually as the radius decreases. (Compare with a pirouette). The pressure in the rotation centre at the base of the CB falls thus accelerating the updraught. The condensation process intensifies releasing more latent heat, further accelerating the process and the pressure may fall more than hundred hPa in the centre. When the rotation has started, a ”trunk” forms at the base of the cloud (the smallest spiral – the highest rotation, causing adiabatic cooling and condensation) which gradually works its way downwards. On coming into contact with the ground, earth and rubbish are sucked into the vortex, which is coloured by its content. The tornado moves along with the general air stream. In USA the large scale wind pattern is often from Southwest towards Northeast when tornadoes form and they move at about 30 KT with an average lifetime of a few minutes resulting in a track of a mile or so. Real life observations have revealed tornado tracks of more than 100 NM, probably caused by multiple tornadoes along the same path.


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The wind inside the tornado itself may reach values close to 270 KT, and when winds of that force pass a building, a suction is generated, which simply pulls off a roof or lifts the whole house. (compare with a wing). The following table illustrates a scale that is used to describe the intensity of the tornado. Professor T. Fujita´s tornado scale

KT Damages KT Damages F 0 35 - 62 Light F 3 137 – 179 Severe F 1 63 - 97 Moderate F 4 180 – 226 Devastating F 2 98 - 136 Considerable F 5 227 – 276 Incredible

A tornado cannot be forecast in the usual way due to its short duration and small scale. From experience we know however, that if the band of precipitation in a CB bends like a comma (according to the radar picture), a tornado usually forms at the narrow end. The best way, however, is to use a Doppler radar system and continuous warnings. When tornadoes are likely to form during the next few hours, a tornado watch is issued by the met office for the region. This alerts the public that tornadoes may develop within a specific area during a certain time period. Once a tornado is spotted – either visual or on a radar screen – a tornado warning is issued by the local National Weather Service office. Radio and TV stations interrupt regular programming to broadcast the warning, and in some communities sirens are sounded. At higher latitudes rare tornado encounters are usually made during a hot summer afternoon. Waterspouts There are two types of waterspouts. The first is developed downwards from a convective cloud formed over warm water surfaces where ordinary towering cumulus clouds are developing. The second type is formed by a rapidly converging airflow over a warm water surface and builds upward from the surface just like a whirlwind. These columns of rotating air are known as waterspouts. Outside southern Florida there may be hundreds of them during the March to October period. During the autumn waterspouts can be seen over the northern Baltic Sea. Waterspouts can sometimes be difficult to discover if there is a shower at the same time, but the lower parts of a waterspout is usually composed of sea spray whipped up from the water surface. Pilots don’t fly in tornadoes more than once. In the case of an inadvertent whirlwind or waterspout encounter the flight conditions are extremely turbulent and structural damage may occur.


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QUESTIONS 1. Some typical clouds associated with a cold front are:

a) stratus and nimbostratus; b) altostratus and cirrostratus; c) cumulus and cumulonimbus.

2. The air ahead of the warm front is colder than the air behind the overtaking cold front

at a:

a) cold front occlusion; b) warm front occlusion; c) stationary front.

3. When crossing a cold front in the northern hemisphere, either from the cold to

the warm, or from the warm to the cold side, the wind shift will be such as to require an alteration in heading to:

a) right b) left c) south.

4. When crossing a cold front at a higher altitude the change in the temperature

and wind direction will be:

a) less than the change at a lower altitude: b) greater than the change at a lower altitude; c) the same as the change

5. Warm frontal weather is associated with a:

a) lowering cloud base. precipitation and reduced visibility; b) lowering cloud base, strong wind and visibility; c) wide band of clouds, thunderstorms and good visibility.

6. When crossing a cold front in the Southern Hemisphere from the warm to the

cold sector there will be:

a) a backing in the wind direction; b) a veering in the wind direction; c) no change in the wind direction.

7. Which weather phenomenon is always associated with the passage of a frontal

system:

a) an abrupt temperature decrease; b) a wind change; c) an abrupt pressure decrease.


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8. Ice pellets at the surface is evidence that:

a) there are thunderstorms present; b) a cold front has passed; c) there is freezing rain at higher altitudes.

9. In a temperate depression, the isobars are straight:

a) Ahead of the warm front. b) Ahead of the cold front. c) Behind the cold front.

10. After the passage of a cold front at Cape Town the wind veers to the NW.

This probably indicates:

a) A col. b) The bad weather is over. c) A family of depressions.

11 . A jet stream may occur in the upper levels if the horizontal pressure gradient

is high and will be found in the:

a) The cold sector. b) The warm sector. c) The warm or cold sector depending on the slope of the isobaric surface.

12. In the northern hemisphere the approach of a warm front is indicated by:

a) South/south-westerly winds. b) Rising pressure. c) North -westerly winds.

13. In an occluding frontal system the air ahead of the warm front is colder than the air in

the cold air mass overtaking the warm air mass. The occluded front will be:

a) A cold front occlusion. b) A warm front occlusion. c) A stationary front.


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CHAPTER 9

THUNDERSTORMS

The way in which cumulonimbus cloud forms was discussed in the previous chapter. It was established that such clouds will only form if a state of marked instability exists within the atmosphere. If such instability exists in an extreme form, thunderstorms can develop. Only cumulonimbus clouds will produce thunderstorms, but not every cumulonimbus will produce the extremes which make flight in or near thunderstorms an uncomfortable and possibly hazardous occupation. Thunderstorms are most likely to occur when:

Instability exists throughout a layer of air which is at least 10,000 feet deep. That is to say, that the ELR will be greater than the SALR throughout this layer.

An adequate supply of moisture is available to form and maintain the cloud.

A trigger action is present.

Thermal trigger actions in the form of insolation or advection were mentioned in the previous chapter, as was orographic lifting. In addition the lifting which occurs when the warm air ahead of a cold front is forced to rise up the steep frontal interface of warm and cold air can trigger thunderstorms. Finally the situation where air is converging to a point and consequently rising (convergence lifting) may act as a suitable trigger. Trigger actions may act alone or in a combination. For example a marked cold front may develop thunderstorm activity on reaching an orographic obstacle.


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Depending on the type of trigger action, thunderstorms may be classified as air mass or frontal storms. With air mass thunderstorms, the primary trigger action will be thermal and may be due to insolation or advection. With insolation as the trigger action the thunderstorms develop most readily in a col or weak depression and of course in temperate latitudes at least, only over the land. The most favourable time of day for these storms to develop is in the afternoon with maximum insolation, and they are most likely to occur during the summer months. Air mass thunderstorms also occur over the sea and now the trigger action is advective, with cold moist air moving over a relatively warm sea. Thunderstorms may form in this way by day or by night. Such storms often form along the coastline, where an orographic trigger reinforces the advective trigger action. Because of the nature of the trigger action, air mass thunderstorms tend to be isolated. Over the land the trigger action is primarily insolation, the sky is therefore unlikely to contain any significant layers of stratiform cloud which could mask the presence of the storm cells. Over the sea the trigger action is advective, and the presence of stratiform layers will not impede the formation of thunderstorm cloud. Now the storm cells may well be embedded in the stratiform cloud, presenting a hazard to aircraft which are not equipped with airborne weather radar. With frontal thunderstorms the trigger action is attributed mainly to frontal uplift. Frontal thunderstorms normally occur only at the cold front, although they may occur at the warm front on rare occasions, or on an occluded front. The storm activity now appears as an advancing line of thunderstorm cloud known as a line squall. Such squall lines may well be 100 nm in length and thus present a significant barrier of hazardous weather. Penetration of such a line squall may prove difficult both because of the distribution of the storm cloud and because of the presence of other frontal clouds within which the thunderstorm cloud will be embedded. An indication of potential thunderstorm development may come from the presence of altocumulus castellatus, which is cumuliform cloud with a base above 8000 ft caused by instability at medium levels. The Life Cycle of a Thunderstorm It is not yet clearly understood why some cumulonimbus clouds produce thunderstorms whilst others do not, It is assumed that the large amount of amount of latent heat which is released as moist air condenses within the cloud provides the energy which is necessary for the storm activity to develop. A thunderstorm will normally consist of several cloud cells in different stages of development. The diameter of individual cells varies from one to five miles, with adjacent cells separated by narrow cloud—filled lanes. Each storm cell has a life cycle of three identifiable stages. The direction of movement of thunderstorms has been found to be close to the direction. of the wind at the 700 mb (10,000 ft) level.


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Development Stage New and growing cells can be recognised by their clearly defined cauliflower shaped top and outline. Development is usually very rapid, perhaps being completed within 30 minutes by which time the top of the cloud may reach the tropopause and beyond, reaching in some cases 40,000 ft in temperate latitudes and 60,000 ft in sub-tropical and tropical regions. The top of a developing cell has been observed to rise at more than 5000 ft/min. The development or cumulus stage is shown ….

Mature Stage The mature stage is marked by the onset of precipitation and downdraughts, and by the top of the cloud taking on a less distinct fibrous appearance due to the presence of ice crystals. The precipitation itself creates downdraughts in the cloud, initially due to friction. The descending air warms at the SALR but the descending ice crystals and super cooled water droplets ensure that the downdraught is maintained. Severe up and down draughts may exist close together reaching speeds in excess of 3000 ft/min. Sharp vertical gusts of 10,000 ft/min have been measured. It is because of the descending air that the freezing level, which was originally higher within the cloud than in the surrounding free air, will lower rapidly. In the next section, which deals with practical meteorology, we will learn to expect icing within cumulonimbus cloud both above and below the published (free air) freezing level. On reaching the ground, the cold dense air of the downdraught spreads out horizontally away from the centre of the storm. As this cold air moves away from the storm it causes squall wind conditions which are often severe, with marked change in wind direction as well as a significant increase in windspeed. The leading edge of this spreading cold air is known as the gust front and may extend up to 20 km from the storm centre or up to 40 km from an organised line of storms. The effects of the gust front may be felt at heights of up to 6000 ft above the ground. The mature stage is illustrated. The average duration of this second stage is in the order of 30 minutes. Microburst When a particularly severe storm occurs a microburst may be produced. A microburst is a highly concentrated and powerful downdraught of air, typically less than 5 km in diameter and lasting from 1 to 5 minutes. Downdraught speeds of 60 kt have been observed in severe microbursts.


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A microburst need not be associated with precipitation , a dry microburst occurs when precipitation evaporates before reaching the ground a phenomenon described as virga such as might happen with a high cloud base. The evaporative cooling in this case enhances the strength of the downdraught. Dissipating Stage This stage commences when the storm has exhausted the local supply of moist air. The downdraughts will by now have spread right across the cloud, and the precipitation will have moderated to a light drizzle. The subsiding air may cause the cloud to dissipate, or alternatively the cloud may spread laterally to form stratocumulus. Small updraughts still persist in the upper part of the cloud which consists almost entirely of ice crystals. The top of the cloud therefore persists, and tends to drift downwind to form the characteristic anvil.

The dissipating stage is illustrated previously. The process of dissipation may take two hours or more. Remember that a thunderstorm cloud will consist of several individual cells. At any time, perhaps one cell will be forming, one active and the remainder dissipating. A problem arises in that the subsiding air from the dissipating cells causes convergence which triggers new cells or reactivates old ones. The storm cloud therefore may become regenerative, providing that sufficient moisture persists in the air. A particular condition of storm re—generation is known as the self—propagating storm. In this case the cell re—generates itself rather than forming new ones or re—generating its neighbours. It occurs when a marked change of wind velocity (either in direction or speed) exists within the deep band of unstable air within which the storm cell grows. The cloud is tilted out of the vertical and at the active stage the precipitation tends to fall outside the boundaries of the cell. The resultant downdraughts are remote from the cell itself and consequently do not cause the cell to progress into the dissipating stage. Should there be a layer of dry air within the band of instability, it is believed that the consequent evaporation greatly enhances the energy levels within the system. So far so good, but where does the lightning come from? The answer of course is that if we knew for sure perhaps we could solve the world’s energy problems. The movement of water droplets within an existing electrical field, coalescence, friction between ice crystals, evaporation and melting of ice crystals, freezing of water drops and sublimation of vapour into ice particles are all known to produce electrical energy.


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For some reason areas of significantly differing potential form within the cell. By the active stage the potential difference is great enough to overcome the natural insulating properties of the air and lightning results. Lightning may occur between two points in the same cloud, between adjacent clouds, or between a cloud and the surface. Hazards Outlined below is a brief summary of the hazards associated with flight in or near thunderstorms. Remember that the best precaution available is absolute avoidance. Make full use of available meteorological briefing facilities, act upon SIGMETS when received, and make full use of AWR or stormscope systems when fitted. Hail

Hailstones form readily in thunderstorm cloud, and large stones can cause significant damage to the leading and upper surfaces of an aircraft. As a rule, the hailstones decrease in both size and intensity towards the top of the cloud, and so penetration, if unavoidable, should be made at high level. Hail should be assumed to exist at any level in a thunderstorm. Stability at or near the tropopause results in the characteristic flattened top of a cumulonimbus and strong upper winds cause the overhang of the anvil from which hail may fall. Flight beneath the overhang should therefore be avoided. Hailstones of up to 5½ inches in diameter have been found at ground level and at 10,000 ft hailstones 4 inches in diameter can be encountered. Hailstones large enough to cause structural damage to aircraft should be expected up to 45,000 ft.


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Icing A subsequent chapter deals in depth with icing, both airframe and engine. Suffice for now to say that the abundance of supercooled water droplets will give rapid accumulations of airframe ice when flying above the freezing level. As the temperature drops the amount of liquid water present diminishes and therefore the risk of icing decreases. The consequence of ice on the airframe will be a spoiling of the aerodynamic shaped smooth surface giving an increase in drag and a loss of lift, as well as an increase in weight. The stalling speed will increase, engine intakes may become restricted, unheated static and pitot sensing devices may become blocked, and ice on aerials may cause loss of communications and navigation aids. With propeller driven aircraft uneven icing on the blades may set up dangerous stresses and to cap it all forward visibility through unheated windscreens will diminish rapidly. In order to avoid these problems penetration, if unavoidable, should be made as high as is possible, so that the temperature is well below -10C. Alternatively penetration should be made below the freezing level, which will be lower within the cloud than outside it, providing that minimum safe altitude considerations, revised with due consideration for the turbulence factor, permit such a course of action. In thunderstorms airframe icing may occur from 0°C down to –45°C, however at the lower temperatures fewer supercooled water droplets exist and the probability of severe icing occurring at temperatures below -30C is very much reduced. The increased weight and reduced control effectiveness due to ice accretion may result in control problems, anti icing or deicing systems should be used to the full if penetration of a storm cell cannot be avoided. Induction system icing should also be considered since thunderstorms form in conditions of high humidity. With turbine engines flame out due to ice ingestion must be anticipated and continuous use of the ignition (in accordance with any limitations laid down in the flight manual) should be employed to reduce the risk of flame out. Lightning Lightning strikes on aircraft are thought to be most likely to occur at levels where the temperature is between -10C and +10C, that is to say within about 5000 ft above or below the freezing level. Providing that the aircraft is properly bonded there should be little damage other than burn marks at the points of entry and exit of the lightning strike. External aerials are of course insulated from the airframe rather than bonded to it. Should lightning strike such an aerial, it is likely that the heat generated across the insulating material will burn off the aerial as effectively as a welding torch. Magnetic compasses will become totally unreliable following a lighting strike. The large deviations observed immediately following the strike will decay fairly rapidly. Smaller but significant residual deviations will however remain for long periods and it will be necessary to check and probably recalibrate the compasses before the next flight Lightning flashes may cause short term partial loss of vision at a time when the pilot needs it least. Antiglare glasses (or sunglasses) should be worn and flight deck lights (or the normal internal lighting) should be turned full up to minimise the effect.


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Again flight through the cloud, if unavoidable, should be made at as high a level as is practical. Gyro magnetic compasses should be switched to the pure gyro mode prior to penetration of the storm activity. Static Electricity The static electricity within the cloud will cause interference with radio communication and navigation systems. VHF systems will not be seriously affected; HF systems will suffer rather more than VHF systems whilst equipments operating in the MF, LF and VLF bands will be most affected. Remember that the ADF receiver will indicate unerringly the centre of the nearest active cell! (set to 250 in the tropics to use the ADF as the poor mans radar!!) The existence of static may first be noticed as noise on HF and MF radio bands, and to a lesser extent on VHF receivers. In heavy static a visible discharge known as Saint Elmo’s fire may be seen, particularly around the edges of windscreens, which is very pretty but does absolutely nothing for your night vision. Turbulence Turbulence is strongest in developing and mature cells. Vertical displacements of 5000 ft have occurred and large roll and pitching motions should be anticipated. Mammatus (that is to say mammary shaped) clouds projecting below cumulonimbus or altocumulus clouds are indicative of strong vertical turbulence. Outside of the storm cell severe turbulence may exist out to a range of between 15 and 20 nm downwind. The presence of lightning cannot be regarded as a reliable guide as to where the strongest turbulence exists. Although a storm must be well developed before lightning occurs it may very well continue in the decaying stage when the turbulence has diminished. Severe turbulence can be encountered several thousand feet above an active storm, particularly when windspeeds of 100 kt or more exist. Accidents involving loss of control and in some cases structural failure have occurred as a result of attempts to regain control or through incorrect flying techniques. Windshear The presence of a thunderstorm is likely to create a high risk of windshear. At low altitude changes in windspeed of as much as 80 kt and changes in wind direction of as much as 90 ْcan occur. Windshear is discussed in the following chapter. Tornadoes A tornado is a concentrated vortex that may extend from the surface well into the cloud. Although they are more common and more severe in the USA they can also occur in the UK, Europe and parts of Africa.


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Tornadoes are a very serious aviation hazard and wind speeds within the vortex have been observed at 200 kt. The tornado may be visible as an extension of the cloud down to the surface, however when the cloud does not extend down to the surface the vortex may cause a funnel cloud to form below a cumulonimbus. Water Ingestion There may well be areas within the storm where the updraught velocity exceeds the terminal velocity of water droplets. In these areas very high concentrations of water may occur and a risk of flame—out or of structural failure of jet engines exists. Guidance for the safest operation of jet engines in these conditions should be given in the flight manual, however avoidance must remain the first priority. Instrument Errors Pressure variations and gusts may result in errors in pressure instruments. Altimeter errors of up to plus c-minus 1000 ft may exist. Near the surface heavy rain indicates the areas within which these areas are likely to occur. Airspeed indicator errors may result from water ingestion into pitot heads. Attitude indicators may not provide sufficient accuracy at large angles of pitch, or may not have sufficient a sufficient range of freedom to cope with the attitudes which might be encountered in areas of extreme turbulence. Magnetic compasses cannot be relied upon after a lightning strike and should be checked as soon as possible. Use of Weather Radar The primary purpose of airborne weather radar (AWR) is to aid thunderstorm avoidance. Guidance on the distances by which thunderstorms should be avoided is given below. It should be noted that radar cannot provide reliable indications as to areas of hailstones within a storm cell, since rain and hailstones produce similar echoes on the AWR. Because of the high rate of growth of thunderstorms, if storm clouds have to be over flown, a vertical separation of at least 5000 ft should be maintained. Flight altitude Avoidance using AWR

0 — 20 000 ft Avoid by 10 nm echoes with protrusions or which show rapid changes in shape. Avoid by 5 nm echoes with sharp edges or which show strong gradients of intensity on iso echo mode (narrow return around the hole)

20 000 — 25 000 ft Avoid all echoes by 10 nm. 25 000 — 30,000 ft Avoid all echoes by 15 nm.

Above 30 000 ft Avoid all echoes by 20nm. Aircraft without a serviceable AWR should avoid by 10 nm any storm which is tall, growing rapidly or has an anvil shaped top.


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QUESTIONS 1. Thunderstorms usually associated with heavy hail showers and destructive winds are:

a) warm front thunderstorms; b) squall line thunderstorms; c) thunderstorms in the dissipating stage.

2. The conditions necessary for the development of thunderstorms are:

a) moist air in the lower layers, instability, lifting action; b) moist air in the upper layers, stability, frontal activity; c) sufficient moisture, instability, static electricity.

3. Which is the most common hazard of lighting strikes on aircraft:

a) fuel is ignited in the tanks; b) structural damage to major components of the aircraft; c) failure of the entire electrical system.

4. What are the indications that downdraughts have developed and a thunderstorm has

reached a mature stage:

a) A gust front occurs, and thereafter precipitation begins to fall. b) The anvil top completes its development, associated with light ram. c) Temperatures drop sharply and pressures increase rapidly.

5. Which thunderstorms generally produce the most severe conditions such as heavy

hail and destructive winds:

a) Warm front thunderstorms. b) Airmass thunderstorms. c) Cold front thunderstorms.

6. When flying in turbulence near a thunderstorm it is important to maintain:

a) Airspeed. b) Altitude. c) Attitude.

7. A particular hazard a pilot should bear in mind when he is aware of a thunderstorm

close to the terminal airfield is:

a) The danger of lightning. b) Bad visibility. c) A sudden change in wind direction.


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CHAPTER 10

TURBULENCE MECHANICAL TURBULENCE Wind blowing over buildings and mountains causing eddies.

LOW LEVEL TURBULENCE This is often caused by convection currents from heating.


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WAKE TURBULENCE Wing tip vortices are generated by all aircraft, the greatest danger exists near heavy aircraft operating at low speed and a high AoA. ( i.e. During take-off and landing. )

In the air: These vortices drift down at + 500 ft/min and level off + 900 ft below the generating aircraft. (Always fly above the flight path of a larger aircraft. ) NB - If you accept a clearance to follow another aircraft you have in sight, the responsibility for wake turbulence avoidance is transferred from the controller to you.


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On the ground: Vortex strength diminishes with time and distance behind the generating aircraft. These vortices tend to remain on the ground and move out at about 5 kts. ( A 5 kt cross-wind causes the upwind vortex to remain on the runway, while the downwind vortex may drift across a parallel runway. ) NB - Beware closely separated parallel runways.

Landing When landing after a large aircraft has landed - stay above its glide path and touch down beyond its touchdown point.

When landing after a large aircraft takes off - touch down well before its rotation point. Minimum separation times are: - Medium behind heavy - 2 mins. - Light behind heavy - 3 mins. Take off Taking off after a large aircraft has taken off - rotate before the large aircraft's rotation point, climb out above or upwind of its flight path. Taking off after a large aircraft lands - rotate beyond its touchdown point.


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Minimum separation times are: - Medium or light behind heavy - 2 mins. - Medium or light behind heavy - 3 mins when using intersection. WIND SHEAR TURBULENCE Associated with any wind shear situation, particularly climbing or descending through an inversion.

CLEAR AIR TURBULENCE ( CAT ) Usually associated with the jet stream and experienced as a " cobble stone " effect. It may also occur as wind flows over the top of a barrier at altitude e.g. a squall line of thunderstorms.


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MOUNTAIN WAVES Standing waves are indicated by the presence of Lenticular clouds or rotor clouds. In the absence of cloud, CAT can be experienced. Favourable Conditions Wind - Constant direction. - Increasing speed with height. - Must blow within 30° of mountain. - 15 Kts for small mountain. - 30 Kts for large mountain. Air - Stable below crest. - Unstable above crest. Mountain - Must not be wider than wave length or it will be cut off.


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WIND SHEAR Wind shear is described as the change in direction and/or speed of the wind in a relatively short distance (vertical or horizontal).

FEATURES CONTRIBUTING TO WIND SHEAR

Elevated runways. Airfield in lee of mountain. Approach path over water (frictional). Not landing into wind (noise abatement). Jet streams. Sea breeze against moderate gradient wind. Marked temperature inversion.

APPROACH TECHNIQUES

Alter speed (increase). Alter configuration (flapless). Higher power setting (less response time for engine). Fly decelerating approach. If the aircraft ahead of you calls wind shear, or goes around for no

apparent reason, wind shear may be present.


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MICROBURSTS These can occur below Cu or Cb clouds if virga or a rain shaft is present. As the rain falls and evaporates, it cools the air around it rapidly which then sinks towards the surface. This may cause down draughts in excess of 6 000'/min and surface winds of 120 kts! Look out for Cu clouds with a wispy appearance and associated virga, dust circles about 1 km in diameter or a rain "foot" below a Cb cloud.


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QUESTIONS 1. A common cause of turbulence, especially at low levels is:

a) convective currents: b) light wind; c) warm cloudless conditions.

2. The meteorological conditions which are, in particular liable to produce marked low

level wind shear are:

a) a low level temperature inversion due to radiation; b) fair weather cumulus clouds: c) warm cloudless conditions.

3. Weather information indicates a strong wind blowing perpendicular to a mountain

range. Other information confirming the existence of mountain waves in the area will be:

a) stations downwind from the mountain range reporting patches of Ac. b) stations up-wind of the mountain range reporting Cu clouds. c) stations on both sides of the mountain range reporting no clouds but

good visibility. 4. Clear-air turbulence is frequently associated with:

a) Jet streams; b) Cb clouds: c) Cold fronts.

5. Wind shear occurs:

a) Only at high altitudes in the vicinity of jet streams. b) At any level and it may be associated with a change in the wind speed, wind

direction or both. c ) Primarily at lower altitudes in the vicinity of mountain waves.


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CHAPTER 11

ICE There are two distinct types of icing which present a hazard to aircraft in flight. The first is airframe icing, and the second is engine icing. Airframe Icing Ice accretion on the surfaces of an aircraft can only occur if the airframe is below 0°c, the ambient temperature is below 0°c and, with one exception, supercooled water is present. Supercooled Water Droplets Water exists commonly in the atmosphere in its invisible vapour form. When air containing water vapour is cooled, it will eventually reach its dewpoint and is now saturated. Further cooling will cause condensation to occur. The process of condensation, or the change of state of water from vapour to liquid form, requires a supply of nuclei. These nuclei, which are termed hygroscopic nuclei, form the core of the condensed water droplets. Hygroscopic nuclei, which may be sulphuric combustion products or sea salt particles, are normally in plentiful supply and so condensation will occur readily when the air is cooled below its dewpoint. Since dewpoint reduces as water vapour content reduces, a point will arise where air which is very dry must be cooled to below 0(c before saturation is achieved. However at temperatures below 0c the water vapour forms into ice before it reaches its dewpoint. The temperature at which this occurs is called the frost point and the process is called sublimation. Ice will only form, however, if a suitable surface or an ice nucleus is present. At ground level this presents no problem, but in the free atmosphere where ice nuclei may be absent, water vapour is unable to form ice and condensation occurs instead. The resulting water droplet, which exists at a temperature below 0(c, is known as a supercooled water droplet (SCWD). These supercooled water droplets are unstable, and any subsequent contact with a surface or an ice nucleus will result in a change of state of the SCWD into ice. Ice nuclei, which are believed to be very small ice particles, are typically only found at lower temperatures. Consequently, in the temperature range 0(c to –10(c where ice nuclei are almost completely absent, SCWDs are abundant. From –10( to –40(c the proportion of SCWDs reduces progressively, until at temperatures below –40(c they are normally absent (except in cumulonimbus cloud). For an aircraft the hazard created by the SCWD is obvious, however the type of ice formed depends on the size and the temperature of the droplet. From chapter one we know that latent heat is always absorbed or released when matter changes state. When water freezes latent heat is released at the rate of 80 calorie per gram of water. This release in latent heat maintains a portion of the SCWD in its liquid state when it impacts on a surface such as the leading edge of an aircraft wing. The proportion of the droplet which remains liquid depends on temperature and is based on the rule that 1/80th part of the droplet freezes into ice instantly for each degree Celsius that the droplet is below zero.


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Due to the release of latent heat, then, a supercooled water droplet at a temperature of -5(C striking the leading edge of the wing will behave in the following way:

5/80ths of the water droplets will freeze on impact with the wing.

The heat energy released at this stage will raise the temperature of the remaining 75/80ths to 0(C.

This water will now flow back over the top surface of the wing, losing heat to the aircraft skin and freezing as it flows back.

Opaque Rime The portion of the supercooled water droplet which freezes on impact with the leading edge does so more or less instantaneously and in so doing will trap pockets of air. The results of this will be that the ice on the leading edge will be whitish in appearance, it will be light and honeycomb in structure, because of the air, and brittle. This ice is known as opaque rime or simply as rime ice and is encountered in clouds of low water content composed of small SCWDs.

� Clear Ice This type of ice exists as a transparent or translucent coating which takes on a glassy appearance. This ice results from the water which flows back over the aircraft freezing as it does so. Droplets combine together whilst still liquid and form a continuous surface which when frozen is dense, heavy and hard to remove. Clear ice forms when large SCWDs are encountered and is worst, for a given droplet size, at temperatures which are only just below zero. At these temperatures only a small part of each SCWD will freeze on impact with the remainder freezing relatively slowly as it flows back over the cold (subzero) aircraft.

�


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Icing in Cloud The type of icing to be found in cloud depends on the cloud type and the method of its formation. Convection clouds are associated with strong vertical currents which can therefore support larger SCWDs and are more likely to result in clear ice formation. Stratiform clouds on the other hand are formed by the turbulence process or by the comparatively gentle uplift of air, for example at a warm front. Such clouds usually consist of smaller water droplets and tend towards opaque rime formation. Individual cloud types can vary quite widely from the generalisation given above and are discussed below. At Temperatures down to –20(C cumulus clouds consist almost entirely of SCWDs, with the greatest number occurring in the most newly formed cloud. Thus in the temperature band 0(c to –20(c (-30(c in cumulonimbus) severe clear icing should be anticipated. Moderate or light icing should be expected at lower temperatures with little or no icing below –40(c (-45(c in cumulonimbus). Stratus usually consists of small water droplets and at temperatures below 0(C may give light to moderate opaque rime. Stratocumulus usually consists of water droplets at temperatures down to –15(c and may cause moderate opaque rime icing. Orographic lifting (discussed shortly) will however increase the severity of the icing experienced. Stratocumulus can also form from the spreading out of cumulus under an inversion, most frequently over the sea in winter. Again the severity of icing in stratocumulus formed in this way may be increased. Altocumulus normally consists entirely of small water droplets at temperatures down to -10C. At lower temperatures the proportion of SCWDs reduces but remains predominant down to -30C. Airframe icing is likely to be light to moderate opaque rime except in altocumulus castellatus and altocumulus lenticularis where convection and orographic effect respectively increase the water content and droplet size in the cloud. Altostratus usually consists of small water droplets giving light to moderate opaque rime. Nimbostratus cloud may extend from a few hundred feet above the surface at a warm front to at least 5000 ft and frequently above 10,000 ft. Some part of the cloud is likely to contain SCWDs large enough to cause clear ice to form. Moderate icing should be anticipated in this cloud between 0c and -15C, however if the front is active, or if there is a significant orographic effect, moderate or severe icing should be expected at temperatures as low as -25C.

Cirrus cloud usually consists entirely of ice crystals and does not therefore pose an icing risk. �


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� Cloud Base Temperature At the cloud base, the air is, by definition, just saturated. The amount of water vapour required to saturate air at a particular pressure varies directly with the temperature. In other words, the higher the temperature at which saturation first occurs, the greater the concentration of water within the cloud thus formed. Air which ascends in a convective cloud carries water droplets upwards, consequently the moisture content of the cloud is much the same at all levels. The water content of a convective cloud therefore increases directly with the temperature at the cloud base, and ice accretion at a given height above the freezing level is liable to occur at a greater rate with a higher rather than a lower cloud base temperature. The icing risk in convective cloud is consequently greater in tropical regions than in temperate regions, and greater in summer than in winter.

Orographic Effect Icing in cloud over high ground is likely to be worse than it would be over level terrain. The forced ascent of moist air from lower levels tends to increase the rate of condensation and as a result the cloud contains more free water. Additionally, the increased upward motion results in more and larger water droplets being retained in the cloud and consequently a greater icing risk. The accelerated rate of cooling which occurs when stable air is lifted orographically tends to lower the cloud base slightly over hills. The 0(c level will also occur at a lower level. Rain Ice One particularly unpleasant type of airframe icing is known as rain ice. It occurs, in temperate latitudes, normally when an aircraft is flying in the cold air ahead of a warm front, during the winter months. For rain ice to form on the airframe the aircraft must be flying above the freezing level. The problem becomes serious when the freezing level is low enough to prevent the aircrafts descent into the warm air below, because of terrain clearance considerations.

Above illustrates the situation in which rain ice will readily occur. The aircraft and the air in which it is flying are both at subzero temperatures and large water droplets are falling out of the nimbostratus cloud at the warm front. If the rain is not supercooled on leaving the cloud it may well become so as it falls through the cold air under the front. Because the airframe is at a temperature which is below 0°c, and because the concentrations of supercooled water


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droplets within the critical temperature band (0° to –10°c) are high, dangerous levels of translucent ice will form on the upper surfaces very rapidly. On encountering rain ice, the pilot has three sensible options:

(i) To turn back.

(ii) To climb into the warm air on the other side of the front.

(iii) To descend in the cold air and fly beneath the freezing level if terrain clearance permits.

Hoar Frost Hoar Frost is a white semi crystalline coating of ice which often appears on the ground, and on aircraft when they are left out in the open during long winter nights, when the skies remain predominantly clear of cloud, and the temperature drops below 0°C. As the surface temperature drops rapidly under the clear skies the surface air temperature will also drop rapidly, especially in still air or light wind conditions, which will inhibit any mixing of the surface air with the warmer air above. If the air is sufficiently dry the dewpoint will be at a temperature which is below 0°c. In this situation sublimation will occur when the air reaches a temperature which is approximately 1°c above the dewpoint. It is at this temperature that the air becomes saturated with respect to ice. This critical temperature is known as the frost point. Sublimation is the change of state of water directly from vapour to solid (ice crystalline) state. The synoptic situations favouring the formation of hoar frost are anticyclones, ridges of high pressure or cols, all of which tend to give the necessary light winds, dry air, and clear skies. Hoar frost must be removed from an aircraft before flight. The presence of a rough ice layer will increase drag, decrease lift, obscure windscreens and interfere with radio navigation aids and communications if it forms on aerials. There will be a slight increase in aircraft weight, and control surface movement could be inhibited. Furthermore, should the aircraft fly through an inversion shortly after takeoff, which is likely in the prevailing meteorological conditions, further frosting will occur which will readily adhere to the already roughened surfaces. The resulting increase in stall speed and loss of lift could have serious consequences at this critical stage of flight. An aircraft which has been flying at high altitudes and subsequently makes a rapid descent may well pick up hoar frost during the descent. This is because, when the aircraft arrives in much warmer air at lower level the airframe may still be cold enough to chill the air flowing over the airframe to its frost point. Ice formation on the airframe with result in: (i) Increase in all up weight, lowering the climb rates and cruise ceilings. (ii) Increase in drag; decrease in lift; decreasing airspeeds and angles of climb;

increasing stall speeds.


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(iii) Possible restrictions of control surface movement. (iv) Reduction of visibility through windscreens. (v) Blockage of unprotected pitot tubes and static vents. (vi) Interference with radio communications and radio navigation aids when ice forms on

aerials.


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Airframe Ice Protection Systems Windscreens, propeller blade leading edges and occasionally wing and tailplane leading edges may be protected with electrical heating elements. Hot air may be taken from the compressor stages of jet engines and fed along pipes behind leading edges to protect against icing. Expanding rubber boots may be fitted to leading edges. The above systems will, to some degree, be effective in removing ice which has already formed. They are therefore termed de icing systems. Systems which pump glycol or alcohol over surfaces requiring ice protection are only really effective if the surfaces are moistened with the appropriate agent before the icing occurs, and are therefore normally referred to as anti icing systems. Engine Icing In some instances the distinction between airframe and engine icing is difficult to draw. For example pack snow, opaque rime or clear ice on the air intake of a piston engine or the nacelle of a jet is airframe icing, however when this restricts the airflow into the piston engine it will affect engine performance, and when it breaks away and enters a jet engine it can damage leading stage compressor blades. Whether the build up which forms on engine intakes is pack snow, opaque rime or clear ice it is often referred to as impact icing. For jet aircraft with rear fuselage mounted engines, the consequence of significant amounts of airframe ice breaking free from the wings can and have resulted in double engine failure and the loss of the aircraft. Ice which forms on a propeller will decrease its efficiency and, if unevenly distributed, set up structural stresses in both propeller and engine crankshaft. On parting company with the propeller ice may also strike the airframe, causing structural damage. Ice which forms over fuel tank vents will cause a drop in pressure within the tanks as they empty and could in the extreme cause engine failure due to fuel starvation. So far the types of icing which have been discussed as affecting engine operation have been conditions of localised airframe icing, and as such will pose the greatest problem between temperatures of 0°C and –10°C, and no problem at all temperatures of +1°C and above or –45°C and below. Fuel will not freeze at normal operating temperatures, although it may start to wax at the low temperatures associated with high altitude flight in unheated tanks. Any water which is present in the fuel will, however, freeze at 0°c, blocking fuel filters and leading eventually to engine failure.


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Carburetor/Induction System Icing Unlike any icing previously discussed carburetor icing may be experienced when flying in clear air at temperatures as high as +30°C. The icing which forms within or adjacent to the carburetor is due to two causes: (i) As fuel is introduced into the airflow at the fuel jets evaporation will occur. The latent

heat of evaporation is drawn from the fuel air mix and the body of the carburetor. (ii) As air passes the throttle valve it accelerates, the pressure drops and the air cools

adiabatically. This cooling effect will be most pronounced when the engine is running at low rpm and the throttle valve restricts a large portion of the airflow orifice.

As a result of the two situations described above ice will build up within the carburetor, reducing the airflow, possibly blocking fuel jets and freezing moving parts such as the throttle valve. The total cooling effect may result in a temperature drop of as much as 30C. During this temperature drop, air which is unsaturated may cool through its dewpoint and condensation will occur. Further cooling will then result in a deposit of ice. The following facts are worthy of note:

Carburetor icing may occur with air temperatures as high as +30C with a closed or partially closed throttle.

The icing problem is most likely to be severe at temperatures of +5C to +15C with a

relative humidity of 60% or greater.


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Jet Engine Icing Finally, we should appreciate that gas turbine engines are not immune from a form of engine icing. Adiabatic cooling can occur at the engine intake where the increased airflow results in a reduction of pressure and a consequent lowering of the air temperature, perhaps by as much as 5°c. If no precautions are taken, this can lead to a build up of ice on the struts, inlet guide vanes and the nose cone, despite the fact that the true outside air temperature is above 0°c. In order to avoid this, it is normally recommended that the engine anti—icing is switched on in the air whenever the ram air temperature is at +6°c or below and visible moisture is present, or on the ground whenever the true air temperature is at +6°c or below and either visible moisture is present or the temperature and dewpoint are reported as being within 3°c of each other. Gas turbine engine antiicing systems normally employ hot air from a compressor bleed, which is then routed through the nose cowl (nacelle), struts, inlet guide vanes and the nose cone (bullet) in order to heat these surfaces. When the aircraft is on the ground with the engines at low power it is likely that the compressor bleed supplying hot engine anti—icing air will not deliver an adequate quantity of air to do the job. It is therefore normally recommended that the engine power be increased periodically in order to keep the intake areas clear of ice. Some gas turbine engines use hot oil, electric heating mats or a combination of the three available methods to achieve ice protection.


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QUESTIONS 1. Clear ice forms as a result of:

a) water vapour turning directly into ice; b) small super cooled water droplets freezing, almost instantaneously; c) large super cooled water droplets spreading as they freeze.

2. The characteristics of rime ice and conditions most favourable for its formation are:

a) opaque, rough appearance. tending to spread back over the aircraft’s surface, most frequently encountered in cumuli form cloud;

b) milky granular appearance, forming on leading edges and accumulating forward into the airstream, encountered in stratiform clouds and temperatures - 10 C to –20° C;

c) transparent appearance and tending to take the shape of the surface on which it freezes, encountered in stratiform cloud at temperatures only slightly below freezing.

3. Convective clouds are the most dangerous from the icing point of view because:

a) convective clouds indicate a low freezing level; b) the strong vertical currents mean that a predominance of small super

cooled water droplets will be present; c) the strong vertical currents mean that a predominance of large super

cooled water droplets will be present. 4. Which minimum temperature range is most conducive to aircraft icing in

stratiform cloud:

a) -2C to -15C; b) 0C to - 15C: c) 0C to -10C. 5. The environment in which aircraft icing is most likely to have the highest rate

of accumulation is:

a) Heavy, wet snow. b) Cu clouds. c) Freezing rain.

6. Hoar frost can form on an aircraft when:

a) Flying through freezing rain. b) Flying through rain above the freezing level. c) On a clear still night with temperatures below freezing point.


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CHAPTER 12

CLIMATOLOGY Climatology is the study of the average weather of the various areas of the Earth. In the study of this subject a good quality world atlas, giving not only topographical information but also maps of normal seasonal temperatures and pressure distribution would be very useful. For the purposes of the examination syllabus the candidate is required to have an understanding of the idealized general circulation of the atmosphere, the modifications to this circulation and the resultant weather features associated with specific areas of the world. The syllabus also requires candidates to have some knowledge of the weather of the main air routes and the upper level winds and jetstreams likely to be met on those routes. This study of climatology will be made easier if you acquire some knowledge of geography, the location of the major cities, countries and areas of the world particularly in relation to the equator and the tropics of Cancer and Capricorn, hence the atlas if you are not already a globetrotter. The starting point is an examination of the idealized circulation. Idealized Circulation If the earth had a uniform surface, and was not rotating, the flow of air (or circulation) would be the simple flow where there would be just air rising at the equator and settling and cooling over the poles. The surface of the Earth receives much more intense insolation at the equator than at the poles. Because of the high surface temperatures which this produces at the equator, the air is warmed, expands and rises creating high pressure at altitude over the equator (compared with the poles), and the outflow of air at height due to this high pressure creates low pressure at the surface. At the poles, due to the low surface temperature, there is subsiding air which produces high pressure at the surface and low pressure at high levels. Consequently there would be a general movement of air towards the poles at high levels due to the high pressure over the equator and the low pressure over the poles. At the surface the flow of air would be from the high surface pressure at the poles towards the low surface pressure at the equator. In fact the flow described above is something like a sea breeze but on a gigantic scale.


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Earth Rotation The effect of the Earth’s rotation and thus coriolis (geostrophic) force on the circulation described above is shown below, the flow of air obeying Buys Ballot’s Law.

At upper levels the air moving towards the poles from the equator is now turned right in the northern hemisphere and to the left in the southern hemisphere by geostrophic force. This takes place at about 30° from the equator because, as we already know, the geostrophic wind equation breaks down near the equator. The resultant accumulation of air creates high surface pressure in latitudes 20° to 40° (in each hemisphere) as well as strong westerly winds at height. The two belts of high pressure are recognisable features of the actual world circulation as the subtropical high pressure belts. As a result of the high surface pressure there is an outflow of air at the surface towards the equator and the poles. The picture of the idealized circulation can now be expanded to show the zones of high and low pressure and the resultant air circulation. The diagram below shows both a plan view and an elevation of the various zones and the horizontal and vertical movement of the air.

Important Climatic Features The following important pressure zones and features result from the circulation shown above.


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Equatorial Low (Equatorial Trough) This is an area of convergence at the surface created by the outflow of air at the upper levels. Air moving towards this equatorial low at the surface is deflected to the right in the northern hemisphere, giving the north east trade winds, and to the left in the southern hemisphere, giving the south east trade winds. The trade winds can become light and variable on entering this area giving the name “doldrums”to the region. The equatorial trough extends to a latitude of approximately 20° into each hemisphere. Subtropical Highs These regions are created by the upper level air being deflected by geostrophic force until it flows almost along the parallels of latitude. The subtropical high pressure belts lie between latitudes 20° and 40° each hemisphere. Trade Winds The trade winds are belts of light winds, typically less than 15 kt, which blow from a fairly constant direction. They are caused by the outflow of air from the subtropical highs moving into the equatorial trough. The NE trades are found in the northern hemisphere and the SE trades in the southern hemisphere. The trade winds extend up to about 10,000 ft. Westerly Winds The outflow of air on the polar side of each subtropical high under the influence of geostrophic force creates belts of mainly westerly winds in both hemispheres. At times these winds can become quite strong particularly in the southern hemisphere between 40ºS and 60ºS where they are called the “roaring forties”. In the northern hemisphere they are found mainly over the oceans and tend to be much stronger in winter than in summer although they can reach gale force in both seasons. Easterly winds The outflow of air from the polar high pressure regions results in the formation of easterly winds at the surface in high latitudes. In reality the polar highs are less well defined than the idealized circulation implies and the polar easterlies can be very variable. Polar Front Depressions The meeting of the subtropical and polar air masses creates a polar front in each hemisphere. Travelling depressions, form on the polar fronts with sufficient frequency for them to appear on mean pressure charts as low pressure areas. A circulation such as that described above would lead to very low temperatures in the polar regions, and conversely very high temperatures at the equator. To maintain the temperature balance over the Earth, great masses of air must move from the equatorial regions to the polar regions and vice versa. These north south surges, of air occur periodically but irregularly and completely distort the theoretical pattern of surface pressure distribution.


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Climatic Zones Because they are essentially products of solar heating, the features previously described would change latitude with the seasonal movement of the sun. Pressure zones, in turn, produce climatic zones. Because rising air cools, much cloud and precipitation forms in the temperate and equatorial belts of low pressure. At the poles and in the sub tropical belts of high pressure, due to subsiding air, conditions are mainly dry. Due to seasonal movements of these systems, transitional zones are produced which are affected by one or other of these systems according to the season. These statements apply to an idealized world with a uniform surface. In the real world the pressure patterns can be much more complex, however this simple approach and the following descriptions of the more important climatic zones will be useful in our study of climatology. Polar Climate High pressure is often replaced by travelling depressions giving unsettled weather and snow. Below 0°c, the ground is permanently covered by ice and snow. Above 0°c, a “tundra” type of climate is found where mosses and lichens grow, although the sub soil remains frozen (permafrost). Disturbed Temperate Climate Here the weather is controlled mainly by travelling frontal depressions and less frequent high pressure systems; winds are mainly westerly, gales are frequent. There is much cyclonic precipitation throughout the year with no dry season. Mediterranean Climate This is a transitional climate, being disturbed temperate in winter and dry subtropical in summer. Winters are cool and unsettled, summers are warm and dry. Arid Subtropical Climate These areas are always under the influence of the sub tropical high pressure belt. As a consequence, skies are clear, it is warm and practically rainless. The great desert areas of the world are found within this zone. Tropical Transitional (Savannah) Climate In winter the weather is governed by the dry trade winds. In summer it is governed by the belt of equatorial rains. The duration of the wet season decreases as latitude increases. Equatorial Climate This zone has two main rainy seasons which occur as the sun crosses the equator but there is no real dry season. There is much convective activity with heavy showers and frequent thunderstorms. Temperature and humidity are both high and are almost uniform throughout the year.


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Modifications to the Idealized Circulation It is now necessary to consider two important aspects which significantly modify the idealized circulation previously considered. Seasonal Variation The seasonal movement of the sun has already been mentioned. The idealized zonal distribution is distorted by the sun as it moves to its northern limit, which is known as the Tropic of Cancer, in June and its southern limit, which is known as the Tropic of Capricorn, in December. On a global scale the confusions between summer and winter in each hemisphere are simplified by considering climate charts only for January and July. Topography So far we have avoided the complications caused by the large land masses and sea areas of the world on this idealized circulation. The fact that the land heats up and cools down more rapidly than the sea accounts for some wide variations from the idealized circulation. The Inter Tropical Convergence Zone The surface pressure charts have shown how the idealized circulation breaks down because of the unequal heating of land and sea areas. Now it is necessary to examine the effect this unequal heating has on the equatorial trough which is fed with the converging trade winds from the two subtropical high pressure belts. Because of these converging winds the region is often called the inter tropical convergence zone (ITCZ), or sometimes the inter tropical front (ITF). We shall be looking at the positions of the ITCZ for January and July, however it must be remembered that the ITCZ will move between these two positions over the intervening six months so that the weather in any region traversed by the ITCZ may vary considerably throughout the year. The weather near the ITCZ is discussed later on in this section.


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In January the sun is in the southern hemisphere and the maximum heating occurs over the land masses which then become areas of low pressure, effectively moving the ITCZ well south of the equator. Over the eastern Pacific and the Atlantic the ITCZ tends to lie mainly to .the north of the Equator. Over the Indian Ocean and the western Pacific Ocean the ITCZ lies in the southern hemisphere. As you can see from the diagram in July (the northern hemisphere summer), the ITCZ reaches its maximum northerly position over China (approximately 45N) rapidly moving back towards the equator over the China Sea. It reaches 20°N over North Africa and southern Arabia and 30° over India. Over the Atlantic and Pacific oceans the ITCZ lies mainly between 10N and 15N. Stability and Moisture Content The trade winds are initially relatively dry and stable, since the air mass originates as descending air in the subtropical high pressure belts. Passage over warmer seas towards the ITCZ produces instability due to heating from below and a rapid increase in moisture content due to evaporation into the dry air at the lower levels. ITCZ Weather ITCZ weather has wide variations depending on local factors. In some areas over land, a relatively narrow front is found, much like the fronts in temperate latitudes. Over the sea, the ITCZ is from 30 to 300 nm wide, with the weather varying from fair weather cumulus with an inversion between 3,000 ft and 8,000ft to heavy cumulonimbus with violent turbulence and severe icing and tops above 50,000 ft at times. Where there are stable conditions at medium and high levels, the cumulus buildup stops and the cloud spreads out into large sheets of stratiform clouds. The main aspect of ITCZ weather is warm moist air with the potential to produce heavy cloud and rain. The ITCZ is also covered in relation to regional climatology in a later part of this section.


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SOUTH AFRICAN WEATHER FACTORS INFLUENCING SOUTH AFRICAN WEATHER The shape of the plateau, warm Agulhas ( E coast ), cold Benguela (W coast ), position of the ITCZ and the upper air westerlies ( 300 hPa) all influence South African weather patterns. SUMMER


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WINTER

THE COASTAL LOW ( Orographic Depression )

A coastal low may develop off the east or west coast. When there is a strong flow of air off the plateau, it will 'remove' air from the coastal region giving rise to a weak low pressure cell. They will move southwards on the West Coast and Northwards on the East Coast. These cells will persist as long as conditions permit. The coastal lows are accompanied by rain, low cloud, and drizzle.


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THE SOUTH WESTERLY BUSTER This is a Summer condition. The Atlantic High ridges in behind the East Coast low. The steep pressure gradient causes very strong winds that will replace the prevailing NE winds. The onset of the wind is very sudden and is a common feature. It may also bring in low St and Sc with rain and drizzle.

THE CAPE DOCTOR

It is also a Summer condition. The SE winds blow away the pollution (DOCTOR). The winds are fairly dry thus mostly clear skies. The table cloth is formed by air being forced to rise over the mountain.


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THE BLACK SOUTH EASTER The Black South Easter is the same as the Cape Doctor, but stronger pressure gradients feeding in moist air to considerable height. If accompanied by a deep low in the interior, widespread rains will occur. (Lainsburg floods, 1981.)


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CHAPTER 13

AVIATION WEATHER FORECASTS Met information available to pilots may be listed under the following headings, abbreviations for messages being shown where applicable:

1. Aerodrome weather reports METAR AND SPECI 2. Aerodrome forecasts TAF 3. Area forecasts ARFOR (not available in Sth Africa) 4. Area QNH forecasts 5. Route forecasts ROFOR (available on request) 6. Sigmet and Airmet information 7. Warnings of hazardous phenomena in the vicinity of airports 8. Warning for aviation sever storms of tropical and sub-tropical origin 9. Aircraft weather reports AIREP 10. Met broadcasts VOLMET & ATIS

ALL MET MESSAGES GIVE THE TIME IN UTC Now lets look at the type of forecasts and observations that you will use in South Africa…


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SOURCES OF WEATHER INFORMATION Usually weather information comes from placing a phone call to the SAWB or by a fax request, but as time goes on we see new ways to ascertain the weather information, some of the latest ways are via:

Vodacom for those so subscribed, The internet, www.sawb.gov.za The internet site of the SAWB has an amazing amount of information :

The aviation section has an amazing amount of information:


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SYNOPTICS-THE SYNOPTIC CHART ( Synoptic means "general survey" )

Isobars and contours:

Most of SA lies on the plateau. above 1 500m. Because of this height difference between coastal and inland stations, too many errors exist if inland stations are reduced to MSL (isobars ). For this reason, isobars ( hPa ) are used over the sea and geo-potential meters (GPM)are used over the land.

To get GPM. the height of the 850 hPa pressure surface above mean sea level is calculated. ( 850 hPa roughly equates to I 500m.) The first figure of the GPM value is then omitted: [Thus the number 508 actually represents a height of 1 508m.]

These height contours in meters are then plotted over the land and represent H and L pressure areas in the normal fashion.

As isobars are used over the sea and GPM over the land. the isobars and the contours don't usually join along the coast.

Inland stations are plotted with pressure in GPM's.

Coastal stations are plotted with pressure in hPa's.

NB Never focus on only one portion of the synoptic chart. First take a general look at the pressure patterns, winds and wx over the entire country before looking at individual station models. Lets look at some feature of the synoptic chart on the next page


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STATION LEVEL DECODE


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155


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Sample station level decodes

1

2

3

0

1

7


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METAR-SPECI-TAF METAR AND SPECI Format for METAR/SPECI is fixed and according to following common presentation: IDENTIFICATION - METAR = routine weather observation - SPECI = selected special weather report - Location identifier. - Date and time indicator in UTC 121100Z for METAR, for SPECI time

group will indicate time of occurrence of change. e.g. METAR FAJS 121100Z ... SURFACE WIND - Wind direction in degrees True rounded off to nearest 10 degs from

which wind is blowing. - VRB will be used in case of variable direction, but to max speed of 3

KT. When used no speed is indicated. - G will indicate max wind speed in gusts. - In calm conditions 0000 will be used. WIND DIRECTION VARIATION 10 minute period preceeding the observation. If wind velocity is greater than

3 kts, the observed two extreme directions shall be given in clockwise e.g. 240V015

VISIBILITY - Minimum horizontal visibility in metres. - Where minimum visibility is less than 1500 m in one direction and

more than 5000 m in another, the direction of maximum visibility will be shown as follows:

7000NE - If visibility equal to or greater than 10 km then 9999 shall be indicated.


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RVR - During periods when horizontal visibility or RVR is less than 1500 one

or more RVR groups shall be included in reports. - For more than one RWY: R03L0900 R21R0600 R21L0400. - RVR values are reported only for touchdown zones of RWY's. - For changes in RVR (tendency over past ten minutes) U = upward, D

= downward, N = No change. Omitted if impossible to determine e.g. R03L0900U.

PRESENT WEATHER - One or more but not more than three groups of codes shall be used to

report all present significant weather at or near airfield. - Only one table of codes is used: ICAO Table 4678.

QUALIFIER

WEATHER PHENOMENA

INTENSITY

1

DESCRIPTOR

2

PRECIPITATION

3

OBSCURATIO

N 4

OTHER

5

- Light Moderate (no qualifier) + Heavy VC In the

vicinity (within 8 km, but not at the aerodrome)

MI Shallow BC Patches DR Drifting

(below 2 metres)

BL Blowing

(extend to above 2 metres)

SH Shower TS

Thunderstorm

FZ Super-cooled

DZ Drizzle RA Rain SN Snow SG Snow grains IC Diamond dust

(ice crystal) vis < 3000 m

PE Ice pellets GR Hail - dia. > 5

mm GS Small hail

and/or snow pellets

BR Mist FG Fog FU Smoke VA Volcanic

ash DU Wide-

spread dust

SA Sand HZ Haze

PO well-

developed dust/sand whirls

SQ Squalls FC Funnel

cloud (s) (tornado or waterspout)

SS

Sandstorm

DS Duststorm


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Examples of the most commonly used groups…. HZ Haze HAZE BR Mist MIST FU Smoke SMOKE DU Widespread dust WIDESPREAD DUST MIFG Shallow fog SHALLOW FOG BCFG Fog patches FOG PATCHES VDFG Fog in vicinity FOG IN VCNTY FG Fog FOG - DZ Light drizzle FBL DRZL DZ Moderate drizzle MOD DRZL +DZ Heavy drizzle HVY DRZL - RA Light rain FBL RAIN RA Moderate rain MOD RAIN +RA Heavy rain HVY RAIN - SN Light snow FBL SNOW SN Moderate snow MOD SNOW +SN Heavy snow HVY SNOW - SHRA Light shower FBL SHWR SHRA Moderate shower MOD SHWR +SHRA Heavy shower HVY SHWR - SHGS Light showers of small hail FBL SHWR OF SMALL HAIL SHGS Moderate showers of small hail MOD SHWR OF SMALL HAIL +SHGS Heavy showers of small hail HVY SHWR OF SMALL HAIL - SHGR Light showers of hail FBL SHWR OF HAIL SHGR Moderate showers of hail MOD SHWR OF HAIL +SHGR Heavy showers of hail HVY SHWR OF HAIL TS Thunderstorm TS - TSSHRA Light thunderstorm with rain FBL TS WITH RAIN TSSHRA Moderate thunderstorm with rain MOD TS WITH RAIN +TSSHRA Heavy thunderstorm with rain HVY TS WITH RAIN - TSSHGR Light thunderstorm with hail FBL TS WITH HAIL TSSHGR Moderate thunderstorm with hail MOD TS WITH HAIL +TSSHGR Heavy thunderstorm with hail HVY TS WITH HAIL - TSSHSN Light thunderstorm with snow FBL TS WITH SNOW TSSHSN Moderate thunderstorm with snow MOD TS WITH SNOW +TSSHSN Heavy thunderstorm with snow HVY TS WITH SHOW FC Funnel clouds FUNNEL CLD +VCFC Funnel clouds in vicinity FUNNEL CLD IN VCNTY VA Volcanic ash VOLCANIC ASH SQ Squalls SQUALLS CAVOK CAVOK CAVOK SKC Sky clear SKC NSW No significant weather NSW


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CLOUDS AND CLOUD HEIGHTS Detailed descriptions of customary cloud types (ST/SC/CU/AC, etc.) have

been dropped. Instead, the FAA approach has been adopted and clouds will in future be described as follows:

FEW (few) will indicate 1 to 2 oktas SCT (scattered) will indicate 3 to 4 oktas BKN (broken) will indicate 5 to 7 oktas OVC (overcast) will indicate 8 oktas SCT060 for 3 - 4 oktas base height 6000’ etc.

- Only two cloud types are regarded as convective, CB and TCU, TCU being a contraction of towering cumulus.

VERTICAL VISIBILITY

If the sky is expected to be obscured and information on vertical visibility is available, vertical visibility VV shall be shown in Units of 30 m (100’) e.g. VV010.

CAVOK - May only be used when: Visibility 10000 m or more. No CB at any level. No other cloud below 5000'. No sandstorm/duststorm/shallow fog/drifting dust, sand or snow. TEMPERATURE / DEWPOINT: - Whole degrees C. - If below 0 deg C preceded by M eg. 18/M04 PRESSURE - QNH value rounded down to nearest hectopascal preceded by Q. Q1012. - When below 1000 hPa, shown with a leading 0 as in Q0998.


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SUPPLEMENTARY INFORMATION This portion of the METAR will start with recent weather indicated by RE.

Only the following phenomena will be reported as recent weather: - Freezing precipitation. - Moderate/heavy rain or snow. - Moderate/heavy ice pellets/hail. - Moderate/ heavy blowing snow. - Sand/dust storm. - Thunderstorms. - Volcanic ash fallout. Also included in supplementary information will be any recorded windshear activity. Shear will only be reported on if recorded along takeoff or approach path to RWY between RWY surface level and 1500' above. Reports as follows: WS RWY03L WS RWY21R Lets look at some real life metars:


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OK 2491 W SAZA40FAPR031800... 102747 776032 000 SAZA40 FAPR 031800 METAR FAAB NIL= FABE NIL= FABL 031800Z 18014KT 9999 RA SCT050TCU FEW100 18/16 Q1020 NOSIG= FABM 031800Z 08010KT 9999 FEW010 SCT080 16/14 Q1025= FABY 031800Z AUTO 14012KT //// // ////// 22/10 Q1016= FACT 031800Z 16014KT 9999 FEW040 25/14 Q1012 NOSIG= FACV NIL= FADN 031800Z 00000KT 3500 RA SCT005 BKN020 BKN120 21/20 Q1020 NOSIG= FADY 031800Z 10010KT CAVOK 25/11 Q1017= FAEL 031800Z 06009KT 9999 BKN025 21/15 Q1021= FAEO 031800Z 12013KT 9999 OVC015 15/14 Q1025= FAER 031800Z AUTO 14002KT //// // ////// 23/22 Q1014= FAGB 031800Z 09007KT 9999 BKN020 20/17 Q1018= FAGG 031800Z 10014KT 9999 FEW010 18/15 Q1016= FAGY 031800Z AUTO 18002KT //// // ////// 15/// Q1023= FAHS 031800Z AUTO 14010KT //// // ////// 23/20 Q1016= FAIR 031800Z 11010KT 9999 -DZ BKN025 SCT080 18/17 Q1022= FAJS 031800Z 12010KT 9999 FEW008 BKN070 16/15 Q1022 TEMPO BKN008= FAKD 031800Z AUTO 11007KT //// // ////// 21/19 Q1019= FAKM 031800Z 10002KT 9999 SCT030CB SCT100 21/19 Q1017 RETSRA= FAKS NIL= FALA NIL= FALI NIL= FALT NIL= FALW 031800Z 18011KT CAVOK 26/11 Q1011= FALY 031800Z AUTO 11005KT //// // ////// 18/16 Q1023= FAMG 031800Z AUTO 14006KT //// // ////// 22/20 Q1021= FAMM 031800Z 01013KT 9999 FEW030CB FEW030 23/18 Q1018= FANC NIL= FANS NIL= FAOB NIL= FAPE 031800Z 09014KT 9999 FEW025 19/15 Q1019 NOSIG= FAPG NIL= FAPH 031800Z AUTO 13006KT //// // ////// 23/21 Q1015= FAPM 031800Z AUTO 17001KT //// // ////// 19/19 Q1023= FAPR 031800Z 05003KT 9999 FEW025 SCT100 21/17 Q1021= FAPS NIL= FARB 031800Z AUTO 16008KT //// // ////// 24/// Q1017 RERA= FASB 031800Z 17013KT CAVOK 27/00 Q1014= FASS NIL= FATH 031800Z 11002KT 9999 OVC018 23/21 Q1016= FAUL 031800Z AUTO 17006KT //// // ////// 20/20 Q1020=


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FAUP 031800Z 20005KT CAVOK 30/04 Q1012= TREND SECTION OF METAR (2HOURS FROM TIME OF OBSERVATION) In trend forecasts attached to METAR reports only three indicators are applied: BECMG - becoming . . . TEMPO - temporary . . . NOSIG - no significant change. Following the appropriate indicator will be the applicable time group for the element reported on. Time group can be further qualified by an AT (at), FM (from) or TL (until) prefix: BECM FM 0930 ... : becoming x for the rest of the period. BECM FM0930 TL1015... : becoming x between 0930 and 1015 and staying x until the end of the 2 hour period. TEMPO FM0930 TL1030 ... : Temporarily between 0930 and 1030

will occur. TEMPO AT0945 ... : x will occur temporarily only at 0945. - NSW may be included to indicate nil significant weather. - NOSIG appended to end of the METAR will indicate just that .. no significant

change expected in the next at least 1 hour. Sample ICAO revised format METAR: METAR FAJS 062100 071005Z 03008G13KT 260V012 2900 1400NW R03L/1500 SCT022 BKN090 16/04 Q1011 WS TKOF03L BECMG FM 1100Z 34005KT 4500 HZ SCT025CB SCT090. Decode as follows: METAR JNB WRITTEN ON THE 6TH AT 2100M FOR THE 7TH FROM 1000 TO 0500Z WIND 030 DEG TRUE 08KT GUSTING 13 KTS WIND VARIATION FROM 260° TO 012° VISIBILITY 2900M REDUCING TO 1400M IN NW RVR RWY03L 1500M INCREASING, SMOKE, CLOUD 3 TO 4 OKTAS 2200' AGL 5 TO 7 OKTAS 9000' AGL TEMPERATURE 16C DEWPOINT 4C QNH 1011 HPA WINDSHEAR TAKE OFF END RWY 03L TREND WEATHER BECOMING FROM 1100Z WIND 340 DEG TRUE 5KT VISIBILITY 4500M IN HAZE 3 TO 4 OKTAS CUMULONIMBUS 2500' AGL 3 TO 4 OKTAS 9000' AGL.


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SPECI (CODES AND CRITERIA FOR ISSUE) SPECI format and conventions are identical to METAR. SPECI may also include a TREND section.

CRITERIA FOR SPECIAL OBSERVATIONS

ELEMENT SPECIAL REPORT TO BE MADE Direction of surface wind When the mean surface wind direction

has changed by 60º or more. Speed of surface wind When the mean surface wind speed has

changed by 10 knots or more. Variation from the mean surface wind speed

When the variation from the mean surface wind speed (gusts) has increased by 10 knots or more.

Horizontal visibility When the horizontal visibility changes or passes any one of the values below: 800 m 1 500 m 3 000 m 5 000 m

Present weather When any one of the following begins, ends or changes in intensity: Tornado or water-spout Thunderstorm Hail Squall Snow and rain mixed Freezing precipitation Drifting snow Duststorm or sandstorm

Cloud base When the height of base of cloud covering more than 4 oktas of the sky changes to or passes any one of the values below: 200 ft 500 ft 1 000 ft 1 500 ft

Cloud amount When the amount of cloud below 1500 ft changes: i) from SCT or SKC to BKN or

OVC. ii) from BKN or OVC to SCT or

SKC


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TAF TAF is the international code for a terminal aerodrome forecast. Any TAF amendment will be prefixed TAF AMD. As a generalisation, TAF coding follows the same conventions as for METAR/SPECI and the same table 4678 is used. TAF MESSAGE FORMAT IDENTIFICATION - Message ID TAF. - ICAO location indicator. - Date and time of origin of forecast UTC 271200Z - Date, beginning and end times in UTC of forecast period 271212 TAF 271200z FAJS 271212 ....... TAF for FAJS issued on the 27th at 1200 ZULU. FORECAST SURFACE WIND Same format as METAR. FORECAST VISIBILITY Same format as METAR.. FORECAST SIGNIFICANT WEATHER - Same table as METAR. FORECAST CLOUD/HEIGHT - Same rules as METAR. - Some TAF reports may only contain cloud regarded as operationally

significant, i.e. cloud below 5000', ignoring higher cloud except of course CB/TCU.


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CAVOK - Same rules apply. - When CAVOK not really appropriate may be substituted with SKC or

NSC. TEMPERATURE - T for temperature in deg C/time Z. - TM10/0100Z ICING - Information presented in the following manner: 620904 where 6 is icing indicator 2 indicates light icing in cloud 090 indicates base height of layer of icing 9000' AGL 4 indicates thickness of layer i.e. 4000' 0 Indicates to top of cloud. TURBULENCE - Presented as: 562508 where 5 indicates turbulence 6 indicates severe turbulence in clear air 250 indicates base height in layer of turbulence 25000'

AGL 8 indicates thickness or layer of turbulence 8000'. SIGNIFICANT CHANGES IN FORECAST CONDITIONS WITHIN FORECAST PERIOD - Probability of change is indicated by means of PROB. TEMPO - temporary fluctuations < 1 hour duration. BECMG - Used to indicate the beginning of a self

contained portion of forecast. BECMG may also be used in conjunction with FM/AT/TL.

- Where FM is used in a forecast as a stand alone statement, i.e. ....

FM14 ...... all conditions before this group are superceded by the conditions indicated after the group FM.


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SAMPLE TAF TAF FAGC 160200Z 160606 13018KT 9000 BKN020 T19/0900Z BECMG 0608 SCT015CB BKN020 541800 TEMPO 0812 17025G40KT 1000 TSRA SCT010CB BKN020 650100 T16/1000Z FM12 15015KT 9999 BKN 020 BKN100 T20/2000Z Transcribed as: Forecast FAGC(African region, South Africa, Grand Central) date of issue 16 time of issue 0200Z Period 16/0600z to 17/0600z SW 130 deg 18Kt / visibility 9000 m / 5 t0 7 oktas cloud 2000' AGL / temperature 19 C at 0900Z / weather becoming between 0600/0800Z 3 to 4 oktas cumulonimbus cloud at 1500' 5 to 7 oktas cloud at 2000' / infrequent moderate turbulence in cloud 18000' AGL to tops of cloud / temporarily between 0800/1200Z wind 170 deg 25 gusting 40 Kts visibility 1000m in thunderstorm rain / 3 to 4 oktas cumulonimbus cloud at 1000' 5 to 7 oktas cloud at 2000' / moderate icing in cloud 1000' AGL to tops of cloud / temperature 16 C at 1000Z / from 1200 surface wind 150 deg 15KT / visibility greater than 10000m / 5 to 7 oktas cloud at 2000' / 5 to 7 oktas cloud at 10000' / temperature 20 deg C at 2000Z. Now that’s how a simple one is done lets now look at some more real life examples for discussion in class:


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International TAFS by Route Corridor:

Malawi-Mauritius-South Africa-Zambia-ZimbabweAvailability of aerodrome forecasts from overseas locations is dependent on regular international exchange data, and for some locations may be absent. FIMP 18:35 UTC, 29/12/1999

TAF FIMP 291815Z 300024 09010G25KT 9999 SCT018 SCT050 PROB30 TEMPO 5000 SHRA FEW009 BKN014

FMEE 16:47 UTC, 29/12/1999 TAF FMEE 291645Z 292106 12015KT 9999 FEW020 PROB30 TEMPO 2101 6000 SHRA SCT020 SCT050 BECMG 0305 11020KT

FLLS 04:52 UTC, 29/12/1999 TAF FLLS 290400Z 290606 0912KT 9999 SCT020 SCT100 FM 1000 9999 FEW035TCU BKN040 PROB40 TEMPO 1117 6000 TRSA SCT030CB BKN090 BECMG 1618 12004KT 9999 FEW040CB SCT100

FWKI TAF not available

FAJS 18:24 UTC, 29/12/1999 TAF FAJS 292100Z 300024 03008KT 9999 SCT010 PROB30 TEMPO 0206 4000 BCFG OVC005 BECMG 0608 36010KT SCT020 BECMG 0810 33012KT FEW035CB SCT040 BKN080 PROB30 TEMPO 1224 4000 TSRA BKN010 SCT030CB T15/03Z T25/12Z

FVHA 17:09 UTC, 29/12/1999 TAF FVHA 292130Z 300024 09010KT CAVOK BECMG 0810 SCT035 PROB30 TEMPO 1216 FEW040CB FM1600 CAVOK

FAWK TAF not available

FABL 17:42 UTC, 29/12/1999 TAF FABL 291800Z 292109 01005KT 9999 FEW060CB BECMG 2123 CAVOK BECMG 0609 34010KT 9999 SCT035

FADN 15:27 UTC, 29/12/1999 TAF FADN 291500Z 291818 20015KT 9999 SCT018 TEMPO 1821 BKN015 BECMG 1921 26005KT BECMG 0608 04018KT FEW025 TEMPO 1418 BKN020 T23/18Z T21/03Z T30/11Z

YPCC 16:25 UTC, 29/12/1999 TAF YPCC 291623Z 1812 11012KT 9999 SCT025 T 26 26 27 29 Q 1009 1008 1010 1010

Or try some local TAFS :


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South African Weather Bureau

TAF FABL 301800Z 302109 26008KT 9999 FEW030CB PROB30 TEMPO 2124 7000 TSRA SCT040CB BECMG 2403 CAVOK BECMG 0609 31012KT 9999 SCT040= FAKM 301800Z 302109 26006KT CAVOK BECMG 0609 31014KT 9999 SCT045= FAUP 301800Z 302109 24008KT CAVOK= FAWM 301800Z 302109 28008KT 9999 FEW030CB PROB30 TEMPO 2124 6000 TSRA SCT035CB BECMG 2402 CAVOK BECMG 0609 31014KT= FACT 301800Z 302109 18015KT 9999 SCT020 SCT050 BECMG 0609 18018KT FEW030 T24/09Z= FAGG 301800Z 302109 28008KT 9999 BKN015 BKN040 PROB40 TEMPO 2109 4000 -RA SCT008 BECMG 0609 15015KT T22/09Z= FALW 301800Z 302109 19015KT 9999 BKN015 BKN040 BECMG 2124 SCT020 BECMG 0609 FEW030 T24/09Z= FADN 301800Z 302109 06012KT 9999 FEW025 BECMG 1820 03008KT FEW018 BECMG 2022 SCT015 FM0000 22015G25KT 4000 HZ BKN015 PROB30 TEMPO 0103 2000 -RA BECMG 0103 24012KT BECMG 0709 20015KT 9999 BKN015 T22/03Z= FAJS 301800Z 302109 32010KT 9999 FEW030CB BKN090 T15/03Z= FAPE 301800Z 302106 25015KT 9999 SCT008 BKN020 PROB30 TEMPO 4000 -RA SCT005 BKN012 T17/03Z= FAEL 301800Z 302106 24012KT 9999 FEW008 BKN020 PROB30 TEMPO 0006 BKN003 T19/03Z= FAUT 301800Z 302106 28012KT 9999 BKN025 PROB30 TEMPO 2000 VCFG SCT007 T17/03Z= FBSK 301815Z 302106 22010KT FEW040CB SCT090 FM 2200 CAVOK= FACT 301500Z 301803 19018KT 9999 SCT020 BKN040 PROB30 TEMPO 1822 4000 -RA BKN012 T20/18Z= FAGG 301500Z 301803 21012KT 9999 BKN015 BKN040 BECMG 1820 28008KT PROB40 TEMPO 1803 4000 -RA SCT008 T20/18Z= FALW 301800Z 301803 21015KT 9999 BKN015 BKN040 BECMG 2024 SCT020 T20/18Z= FACT 301500Z 301803 19018KT 9999 SCT020 BKN040 PROB30 TEMPO 1822 4000 -RA BKN012 T20/18Z= FAGG 301500Z 301803 21012KT 9999 BKN015 BKN040 BECMG 1820 28008KT PROB40 TEMPO 1803 4000 -RA SCT008 T20/18Z= FALW 301800Z 301803 21015KT 9999 BKN015 BKN040 BECMG 2024 SCT020 T20/18Z= FADN 301500Z 301803 06012KT 9999 FEW025 BECMG 1820 03008KT FEW018 BECMG 2022 SCT015 FM0000 22015G25KT 4000 HZ BKN015 PROB30 TEMPO 0103 2000 -RA BECMG 0103 24012KT T25/21Z T23/03Z= FABL 301500Z 301803 31010KT 9999 SCT030 FEW030CB PROB30 TEMPO 1821 7000 TSRA SCT040CB BECMG 2123 26005KT CAVOK= FAWM 301500Z 301803 32012KT 9999 SCT030 FEW030CB PROB30 TEMPO 1822 6000 TSRA SCT035CB BECMG 2224 26006KT CAVOK= FAJS 301500Z 301803 26010KT 9999 FEW040CB BKN090 PROB30 TEMPO 1822 4000 TSRA BKN010 BECMG 0002 32010KT T15/03Z= FAPE 301500Z 301803 25015KT 9999 SCT008 BKN020 PROB30 TEMPO 4000 -RA SCT005 BKN012 T20/18Z= FABL 301200Z 301524 31010KT 9999 SCT030 PROB30 TEMPO 1521 7000 TSRA SCT040CB BECMG 2123 26005KT CAVOK= FAKM 301200Z 301524 32012KT 9999 SCT045 PROB30 TEMPO 1518 6000 TSRA SCT040CB BECMG 1921 26006KT CAVOK= FAJS 301200Z 301524 22010G20KT 9999 SCT040CB SCT045 PROB30 TEMPO 1524 4000 TSRA BKN010 T16/18Z= FAPE 301200Z 301524 23030G40KT 9999 SCT020 BECMG 1719 25015KT SCT008


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BKN020 PROB30 TEMPO 1724 6000 -RA SCT008 T19/18Z= Location indicator used on TAF and METARS FAAB Alexander bay FAPR Pretoria (Head Office) FABE Bisho FAPS Port Shepstone FABL Bloemfontein Airport FARB Richards Bay FABM Bethlehem FASB Springbok FABY Beaufort West FASS Sishen

FACT Cape Town Int Airport FATH Thohoyandu

FACV Calvinia FATI Thabazimbi FADN Durban Int Airport FATZ Tzaneen FADY De Aar FAUL Ulundi FAEL East Londen FAUP Upington FAEO Ermelo FAUT Umtata FAER Ellisras FAVB VRyburg

FAGB Pietersburg Gateway airport FAVG Durban (Virginia airport)

FAGG George Airport FAVV Vereeniging FAHS Hoedspruit FAVY Vryheid FAIR Irene FAWI Witbank

FAJS Johannesburg Int. Airport FAWK Waterkloof (SAAF)

FAKD Klerksdorp FAWM Welkom FAKM Kimberly FBSK Gaberone FAKS Kroonstad FDMS Manzini (Matsapha) FALA Lanseria Airport FQMA Maputo FALI Lichtenburg FVHA Harare FALT Louis Trichardt FXMM Maseru FALW Langebaanweg FYGF Grootfontein FALY Ladysmith FYKT Keetmanshoop FAMG Margate FYWB Walvis Bay FAMM Mmabatho FYWH Windhoek (Town) FANC Newcastle FYWW Windhoek Int. Airport FANS Nelspruit FAOB Overberg

FAPE Port Elizabeth Airport

FAPG Plettenberg Bay FAPH Phalaborwa FAPM Pietermaritzburg


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CHARTS ABBREVIATIONS FOUND ON CHARTS AND FORECASTS:

ABBREVIATION MEANING

ESC escarpment

LOC local

CIT city

LYR layer

HIV highveld

LOV lowveld

COT coastal

MAR maritime

MON mountains

VAL valley

ISOL isolated individual CB cells

OCNL occasional well separated CB cells

FRQ frequent little or no separation between CB cells

EMBD embedded

FEW few

SCT scattered

BKN broken

OVC overcast

MTW mountain wave

TURB turbulence

LSQ line squall

WS wind shear


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THE SIGNIFICANT WEATHER PROGNOSTIC CHARTS

NB - The word "prognostic" means forecast. Three of these charts are printed: MSL - FL100 LOW ( Cloud bases AMSL ) FL100 - FL250 MEDIUM ( Cloud heights ABOVE 1013.25 hPa ) ABOVE FL250 HIGH ( Cloud heights ABOVE 1013.25 hPa )

Codes used on the chart are given below the Time/Temp/QNH/Wind box.

The symbols for significant weather are given in box 1.

The symbols for fronts, convergence zones, tropopause heights, pressure centres, freezing levels, the ITCZ and the jet stream are given in box 2.

All clouds are denoted within the “mmmmmm” sign.

The symbol XXX denotes that the cloud base (or tops) do not appear on the chart in

use.

Pay attention to the forecast period and the additional information box.


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UPPER WINDS AND TEMPERATURE CHARTS: Two of these fixed time prognostic charts are printed: One from the surface to FL240, the other from FL210 to FL450. Both charts show FL, wind direction (True), wind speed (kts) and temp (° C) Interpolate for winds between FLs shown.


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UK SYNOPTIC CHARTS AND ISOTACH ANALYSIS CHARTS Significant weather chart Next is an example of a Frankfurt Significant Weather, Tropopause/Max. Wind Chart. The chart covers a considerable area of Europe, the Middle East, North Africa and Asia. These charts are issued in advance of their valid times, which are 0000, 0600, 1200 and 1800 UTC.

a. A Polar Stereographic or Mercator projection is used for all middle and upper air significant weather charts. NB Great care must be taken when measuring direction on all small scale meteorological charts. A square navigation protractor must be used. b. The top right hand corner of the chart gives a box which:

i. Indicates the issuing station ii. The type of chart iii. The depth of the atmosphere covered. In this case FL 250 - 450 or FL 370 - 150 HPA. iv. The chart is a fixed time chart for 0600 UTC, 24 February.

NB For the exam the chart can be assumed to be valid for the whole period of flight. No movement of weather phenomena is necessary.

v. Note the statement at the bottom of this box. The symbols and CB imply moderate or severe turbulence and icing. Note: This will be stated in the correct answer required. vi. The units used on the chart are Pressure Altitude (Hectofeet), knots and ° C

c. The vertical distance at which phenomena are expected are indicated by flight levels, top over base or top followed by base. 'XXX' means the phenomenon is expected to continue above or below the vertical coverage of the chart. d. The surface positions together with the direction and speed of movement of pressure centres and fronts are denoted as shown on the chart. Note: Slow indicates movement of less than 5 knots. e. Dashed lines denote areas of CAT. These are areas are numbered and are associated with the decode box on the chart.

eg Area 5 Moderate turbulence FL 460 to below FL 250 (XXX) with localised severe turbulence

f. On lower charts the 0° C Isotherm will also be shown as a dotted line with the FL indicated.

eg - - - - - - - 0°C:FL130 - - - - - - - -


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Upper wind and temperature charts Issued in conjunction with the significant weather chart. These charts give spot winds from 700 mb (FL100) up to 200 mb (FL390). Spot values of wind and temperature are shown at regular intervals (chart 5) of latitude and longitude. The temperatures given are assumed to be negative unless prefixed by PS. The wind arrow symbology is exactly the same as that for the synoptic chart.


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Averaging Wind Velocities Common sense needs to be applied. For instance if we have an east/west track with a wind velocity of 150º/20 kt to the north and 210º/20 kt to the south then the average wind could be said to be 180º/20 kt. In this case numerical averaging is the common sense way of approaching the problem. Example Average the wind velocity and temperature between Point A and Point D

STEP 1 Point A lies between two winds both 270º/100 kt, therefore take the wind at point A to be 270º/100 kt, Temperature -57ºC. STEP 2 Point B tends towards the southern wind so the speed will be more likely to be 65 kt, measure the direction using your protractor, approximately 26Oº/65 kt, Temperature -58ºC. STEP 3 Point C is between two wind velocities but tends slightly to the southern one. So take the wind velocity as 235º/50 kt. Temperature is between -64º C and -62º C. So take the temperature as -63º C. STEP 4 Take the spot wind at point D, 265º/40 kt, Temperature -63º C.

STEP 5 The average wind is the mathematical sum of the winds divided by

4. 260º/65 kt, Temperature -60º C The above problem is quite a simple way of arriving at the mean wind velocity


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On certain routes the averaging process may not be so easy. Suppose that the wind directions that we extrapolate are 100º, 080º, 245º, 255º, 260º, 270º, 285º, 285º. Note that two winds are easterly in origin and opposite to the rest of the extrapolated winds. If we average these values then the mean wind direction calculated would be 220º. Which goes against what is obviously a predominantly westerly wind. In this type of case discard the two easterly winds and only average the westerly wind, coming out with a much more sensible solution of 265º. When averaging the wind make sure that the difference in values of the wind directions that you are averaging is less than 180º. The wind speed is resolved in a similar fashion. If the winds are all from directions within 1800 then a simple averaging process can be used, If as in the above case we have 2 easterly winds and 6 westerly winds then we have to accept that the easterly winds will offset the values of the westerly winds. In this case give positive values to your westerly winds and negative values to your easterly winds eg -45, -50, +20, +30, +65, +65, +70, +90 so our mean wind speed would be 30 kt. Example Use the charts given above to calculate the mean wind velocity and temperature along the track AD.


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Solution

STEP 1 Select winds at suitable intervals along the track. Every 10º is reasonable.

80º W 70º W 55º W 40º W

320º/20 KT 250º/45 KT 225º/40 KT 020º/60 KT

-46º C -47º C -48º C -49º C

30º W 20º W 10º W 0º E/W

030º/30 KT 215º/70 KT 300º/30 KT 290º/40 KT

-46º C -52º C -48º C -48º C

STEP 2 The wind is mostly westerly so ignore the values for 30º

and 40º W. Average the remaining winds. 270º

STEP 3 Give negative values to the easterly wind speeds and then

average all the wind speeds. 20 kt

Wind velocity 270º/20 kt

STEP 4 Average the temperature values

-48ºC


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Compare this satellite picture with the following forecasts

ATP MET FLIGHT TRAINING COLLEGE ATP DOC 9 Revision: 1/1/2001 Version 7

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LIVE RADAR PICTURES: These are available on the SA weather bureau web site and cover basically the whole country, these are included for information only, and are not part of the examination, but are in the syllabus..


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CURRENT WEATHER PICTURES These are also available in different sizes, see if you can match the pictures to the forecast weather in the previous charts…


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GLOSSARY OF COMMON TERMS USED IN METEOROLOGY

A Adiabatic Process A thermodynamic change in the state of a system in which there is no transfer of heat or mass across the boundaries of the system. In a adiabatic process compression always results in warming, expansion in cooling. i.e. subsidence will result in warming and rising of air will result in cooling.

Advection The horizontal movement of an air mass that causes changes in the physical properties of the air such as temperature and moisture.

Advection Fog A type of fog caused by the advection of moist air over a cold surface and the consequent cooling cooling of the air to below its dewpoint. As happens along the Cape west coast.

Aircraft Icing ( Airframe icing) The accumulation of ice on the exposed surfaces of aircraft when flown through supercooled water droplets (cloud or precipitation).

Air Drainage General term for gravity-induced, downslope flow of relatively cold air.

Air Mass It is a widespread body of air that is nearly homogeneous in its horizontal extent, particulary with reference to temperature and moisture distribution; is addition the vertical temperature and moisture variations are approximately the same over its horizontal extend.

Atmospheric pressure(barometric pressure) The pressure exerted by the atmosphere as a consequence of gravitational attraction exerted upon the "column" of air lying directly above the point in question.

B Barometer Instrument for measuring atmospheric pressure.


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Beaufort scale A numeric scale used to estimate the force of the wind when no instruments are available:

Wind speed(km/h)

Designation Description

< 2 calm smoke rises vertically, trees do not move

2-5 light air smoke drift indicates wind direction

6-11 light breeze weather vane moves, leaves rustle

12-19 gentle breeze leaves and twigs in constant motion

20-29 moderate breeze dust and loose paper raised, small branches move

30-38 fresh breeze small trees sway

39-50 strong breeze large branches move, wind whistles wires

51-61 moderate gale whole trees move, walking affected

62-74 fresh gale twigs brake of trees, walking difficult

75-86 strong gale slight structural damage occurs, branches brake

87-100 whole gale trees uprooted, considerable structural damage

101-118 storm widespread damage

119+ hurricane severe and extensive damage

Berg Wind A hot dry wind blowing off the interior plateau of South Africa, roughly at right angles to the coast. Occur mainly in winter when there is a low pressure system south of the country and a strong high over the country

Black Frost A dry freeze with respect to its effects upon vegetation, that is, the internal freezing of vegetation unaccompanied by the protective formation of hoarfrost.

Blocking High Any high that remains nearly stationary or moves slowly , so that it effectively "blocks" the movement of migratory lows(cyclones) across its latitudes.

Buster A sudden shift in wind direction behind a coastal low from north-east to south-west. The buster is well known for its sudden onslaught with winds going from calm to 40 knots+ in a matter of minutes.

C CAT Clear air turbulence. Turbulence experienced by aircraft as it is flying in cloudless conditions


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Cloudburst Any sudden and heavy fall of rain, almost always of the shower type. Most of the times it is associated with thunderstorms.

Cold Front Any non-occluded front that moves so that the colder air replaces the warmer air; i.e. the leading edge of a cold air mass.

Condensation The physical process by which water vapor becomes liquid or solid.

Cut-off Low A cold low which has become displaced out of the basic westerly current, and lies to the south of this current.

Cyclone(low or depression ) An area of low pressure with a cyclonic flow. ( clockwise in the Southern hemisphere).

D Dew Point The temperature to which a given parcel of air must be cooled at constant pressure and constant water-vapor content in order for saturation to occur.

Doppler Weather Radar A new Weather Surveillance Radar system developed in 1988. This powerful and sensitive Doppler system generates many useful products for meteorologists, among them: standard reflectivity 'echoes', wind 'velocity' or atmospheric air motion pictures, and Arial 1-hour, 3-hour , or storm-total precipitation images. Downburst(microburst) A strong downdraft, initiated by a thunderstorm, that includes an outburst of damaging winds on or near the ground. Downbursts may last for anywhere from a few minutes in small scale microbursts on up to 20 minutes in lager , longer lived microbursts. One example of a downburst, called straight-line winds, can reach speeds of 176km to 240km, or squarely in the range of a strong tornado. Downbursts are further detailed as either: Microburst: a convective downdraft with an affected outflow area of less than 4 km wide and peak winds lasting less than 5 minutes. They can create dangerous vertical/horizontal wins shears which can adversely affect aircraft performance and cause property damage.

Dryline A narrow zone of extremely sharp moisture gradients. Thundershowers usually develop just to the east of the dryline in South Africa.

Dust Devil A well developed dust whirl; a small but vigorous whirlwind usually of short duration, rendered visible by dust, sand and debris picked up from the ground. Diameters range from about 3m to 30m; their average height is about 200m but a few has been observed as high as 2000m.

E El Nino Significant warming of the waters in the eastern Pacific Ocean, usually off the coast of South America, which results in shifts of world-wide weather patterns. Can cause prolonged periods of drought or floods. In South Africa, El Nino is associated with prolonged droughts.


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F Fetch An area from which waves are generated by a wind that is nearly constant in direction and speed.

Flash Flood A flood that rises and falls quite rapidly with little or no advance warning, usually as the result of intense rainfall over a relative small area.

Freezing Level In aviation terminology, then lowest altitude in the atmosphere over a given location , at which the air temperature is 0 degrees C; the height of the 0 C constant temperature surface

Fujita Scale A scale used to classify tornadoes based on wind damage and was developed by Theodore Fijita( university of Chicago)

F scale

Wind Speed (km/h)

Damage

F0 64-115 light

F1 116-179 Moderate

F2 180-251 Major

F3 252-329 Severe

F4 330-416 Devastating

F5 417-508 Incredible

G Greenhouse Effect The heating effect exerted by the atmosphere upon the earth by virtue of the fact that the atmosphere absorbs and remits infrared radiation.

Gust Front The leading edge of a mass of relatively cool, gusty air that flows out of the base of a thunderstorm cloud and and spreads along the ground ell in advance of the parent thunderstorm cell; a mesoscale cold front. A shelf or roll cloud may accompany the gustfront.

H Halo Rings or arcs that encircle the sun or moon which are caused by the refraction of light through ice crystals that make up high level clouds.

Heat Thunderstorm A thunderstorm of the airmass type which develops near the end of a hot, humid, summers day.

Heat Wave A heat wave exist when for 3 days the maximum temperature is 5 degrees higher than the man maximum for the hottest month.


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High ( anti-cyclone) An area of high pressure with a anticyclonic circulation. ( anti clock wise in the southern hemisphere)

Hoarfrost A deposit of interlocking ice crystals formed by direct sublimation on objects. Most of the frost experienced in winter on the high lying areas of South Africa is hoarfrost.

Hurricane See tropical cyclone.

I Instability A state of the atmosphere in which the vertical distribution of temperature allows rising warm air to continue to rise and accelerate.. This kind of motion is conducive for thunderstorm development.

Inversion A situation where the temperature increases with hight instead of decreasing. It is quite common in the winter and because there is no upward motion of warm gases it results in severe pollution of the lower layers.

Isobar A line of equal barometric pressure as shown on a weather map.

J Jet Stream A narrow band of strong winds in the atmosphere that controls the movement of high and low pressure systems and associated fronts. Wind speeds can reach 380 km or higher in certain cases. Jet streams are usually found at 30 to 40 000 ft above the surface. It owes it existence to the large temperature contrast between the polar and equatorial regions.

K Knot Unit of speed used in aviation and marine activities to measure the speed of the wind. It is equal to about 1.15 statue mile ore 1.84 km per hour.

L Land Breeze A coastal breeze blowing from land to sea., caused by temperature difference when the sea surface is warmer than the adjacent land. Normally occurs in the early mornings.

La Nina La Nina is characterised by unusually cold ocean temperatures in the eastern equatorial Pacific. It is the opposite of El Nino. La Nina is associated with above normal rain over the summer rainfall areas of South Africa.

M Mesoscale Dimensions of an atmospheric layer that ranges from a few kilometres to some tens of kilometres horizontally and, vertically from the ground to the top of the friction layer.

N Numerical Forecasting The forecasting of the behavior of the atmospheric disturbances by the numerical solution of the governing fundamental equations of hydrodynamics, subject to observed initial conditions;computers and sophisticated computational models are required.


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O Orographic Lifting The lifting of an air current caused by its passage up and over mountains or escarpments.As the air is forced upwards it cools and if moist enough clouds can form and additional cooling results in rain. It can cause extensive cloudiness and increased amounts of precipitation in higher terrain.

Ozone A nearly colourless(but faintly blue) gaseous form of oxygen, with a characteristic odour like that of weak chlorine. It is usually found in trace amounts in the atmosphere, but is primarily found at 30 000 to 150 000 feet. Its production results from photochemical process involving ultraviolet radiation. Because it absorbs harmful ultraviolet radiation at those heights, it is a beneficial gas.

P Precipitation Any or all of the forms of water particles, whether liquid or solid, that fall from the atmosphere and reach the ground.

Q Quasi-stationary Front A front which is stationary or nearly so.

R Radiation Fog It is fog that form over land due to heatloss through radiation during the night and resulting in the cooling of the air to below its dewpoint.This fog generally form in the early morning and dissipate when the sun is warms up the ground.

Rainbow An arc that exhibits the concentric bands of colours of the spectrum and is formed opposite the sun by refraction and reflection of the sun's rays in raindrops.

Rainshadow Areas on the leeward side of a mountain or mountain range which often receive less rain than the windward side. The Klein Karoo is a good example of this.

Relative humidity The ratio of the amount of moisture in the air to the amount which the air could hold at the same temperature and pressure if it were saturated; usually expressed in percent.

Ridge An elongated area of high pressure in the atmosphere: the opposite of a trough.

Roll Cloud A turbulent cloud formation that resembles a roller. This cloud can be found in the lee of some mountains. The air in the cloud rotates around a axis parallel to the range of mountains. It is also sometimes found along the leading edge of a thunderstorm, formed by the rolling action in the wind shear region between cool downdrafts and warm updrafts.

S Severe Thunderstorm A thunderstorm that produces either of the following: damaging winds of 93 km/h or greater, hail 1.9 centimetre in diameter or larger, or a tornado. Severe thunderstorms can result in the loss of life and property.


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Sleet Describes the solid grains of ice formed by the freezing of raindrops or the refreezing of largely melted snowflakes.

Smog A natural fog contaminated by industrial pollutants, literally, a mixture of smoke and fog.

Snow A steady fall of snowflakes for several hours over the same area.

Southern Oscillation A periodic, large scale atmospheric oscillation of the large scale distribution of sea level pressure, and air and water temperature that originates over the southern hemisphere. Consequently, there is an associated change in the surface winds, and some storms become stronger than normal. This oscillation is on the scale of a year or a few years, and has global implications such as widespread drought or flooding. Oceanic fishing is also disrupted.

Squall Line A broken or solid line of thunderstorms that may extend across several hundred kilometres

Subsidence A descending motion of air in the atmosphere, usually with the implication that the condition extends over a rather broad area.

T Thermal A relatively small-scale, rising air current produced when the earth's surface is heated. Thermals are a common source of low level turbulence for aircraft.

Tornado A violently rotating column of air, usually pendant to a cumulonimbus, with circulation reaching the ground. The visible cloud may not reach the ground, but if the lower circulation, marked by dust, dirt, and/or debris, reaches the ground, it is classified as a tornado. It nearly always starts off as a funnel cloud and may be accompanied by a loud roaring noise. Tornadoes are classified into 3 main groups: weak- wind speeds up to 170 km/h: strong- wind speeds of 170- 330 km/h; violent- wind speeds of 340 to perhaps 500 km/h. Tropical Cyclone A cyclone originating over tropical or subtropical waters with organized convection and a definite cyclonic surface wind circulation. Tropical cyclones are large and span areas of 1000 of kilometres. They can causes a great deal of damage when they make landfall. A lot of damage is caused by the storm surge that result in widespread flooding Tropical cyclones get their energy from the warm oceans and therefore dissipate rapidly as they move in over land.

Tropical or Subtropical Depression Cyclones that have maximum sustained winds of 33 knots or less. These are referred to as low pressure systems in public advisories and statements.

Tropical Disturbance An area of organized convection which originates in the tropics or subtropics and maintains it identity for 24 hours or more. In successive stages of intensification, it may be subsequently classified as a tropical wave, tropical depression, tropical storm or tropical cyclone Tropical Storm


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Tropical cyclone that has maximum sustained winds from 34 to 63 knots inclusive. Trough An elongated area of low pressure in the atmosphere, the opposite of a ridge

U Upper-Level Disturbance A disturbance of the flow pattern in the upper atmosphere, which is usually associated with clouds and precipitation.. This disturbance is characterised by distinct cyclonic flow, a pocket of cold air; and sometimes, a jet streak. These features make the air aloft more unstable and conducive to clouds and precipitation.

UVB A biologically effective portion of solar ultraviolet radiation that reaches the earth's surface; in the wavelength range of 280 to 320 nanometres; responsible for sunburn and skin cancers.

V Virga Wisps or streaks of rain or snow falling out of a cloud, but evaporating before reaching the ground.

W Waterspout Very similar to a tornado with the difference that a waterspout occurs over a body of open water. Wet Bulb Temperature The temperature an air parcel would have if cooled to saturation at a constant pressure by evaporation of water into it Wind Chill An apparent temperature that describes the combined effect of wind and low air temperature on exposed skin.

Z Zonal flow

The flow of air along a latitude circle; more specifically the latitudinal(east or west) component of existing flow.


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QUESTIONS SYNOPTIC CHART QUESTIONS 1. A cross in the middle of the station model on a synoptic chart indicates:

a) The sky is obscured. b) There is no cloud. c) There is 9 oktas of cloud.

2. The isobars on a synoptic chart indicate that the winds at a place on the coast

of SA should blow in a westerly direction. If clear skies prevail, the day’ time wind may be expected to have a:

a) South-westerly component and the night wind to be light westerly. b) South-westerly component and the night wind to stronger from the

Southwest. c) South-westerly component and the night wind to veer towards the

Northwest.

3. Windhoek ( station model no. 114 ) is indicating a pressure of:

a) 1011.4hPa. b) 1011.0hPa. c) 1001.7hPa.

4. Station model no. 024 ( East of Windhoek ) is indicating a cloud base of:

a) 2 000 to 3 000’ b) 3 000’ to 5 000’ c) 6 000’ and above.


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5. Station model no. 494 ( NE of Durban ) is not reporting. If it were, it would

indicate a wind direction of:

a) 220° b) 070° c) 280°


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6. Station model no. 112 (W of Cape Town) is indicating 8 oktas cloud cover. This is a result of:

a) Warm moist air being advected from the NW. b) Uplift of air ahead of the approaching cold front. c) Low level radiation fog.

7. Cape Town is showing a maximum horizontal visibility of:

a) 16km. b) 1 600m. c) 1 600’.

8. Port Elizabeth ( station model no. 842 ) is showing:

a) A pressure increase of 6 hPa over the last 3 hours. b) A pressure decrease of 6 hPa over the last 3 hours. c) A pressure increase of 0.6 hPa over the last 3 hours.

9. Port St. Johns ( station model no. 674 ) is reporting low cloud as follows:

a) 5 oktas Sc with moderate or large Cu. b) 7 oktas Sc and small Cu. c) 5 oktas Sc formed by the spreading of Cu.

10. Based on the synoptic situation, the temperate cyclone will reach Cape Town(chart

below):

a) Within three days. b) Within two days. c) Within one day.


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11. The QNH at Durban (Chart below) three hours preceding the time of observation

is:

a) 1018 hPa b) 1016 hPa c) 1017.1 hPa


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TAF/METAR QUESTIONS 1. The coded aviation routine weather report from an airport is called a: TAF

a) METAR b) SPECI

2. Runway visual range will only be measured at an aerodrome if the

meteorological visibility is: a) less than 10 KM; b) 5 kmh or less c) 1500 M or less.

3. SPECI messages are:

a) Routine weather reports for an aerodrome. b) Aviation special report giving information regarding changes in

weather conditions at or near the aerodrome. c) Special messages giving information regarding the conditions of

changes in the meteorological forecast service at an aerodrome. 4. CAVOK means:

a) Viz. greater than 10000. cloud base greater than 5000’, no thunderstorms, no fog, no significant wx.

b) Viz. greater than 10 000m, cloud base greater than 10 000 or more, no Cb, no fog. no significant wx.

c) Viz. greater than l0 000m, no cloud below 5000’ or min. sector altitude, no Cb, no fog. no significant wx.

5. A SIGMET is issued for:

a) A deterioration of wx at an aerodrome. b) Adverse wx which may affect aviation safety. c) A significant change in the forecast wx.

6. Aviation wx reports are given in:

a) Coded form. b) Code and abbreviated plain language. c) Plain language.

7. A change in visibility, low cloud, surface wind or wx is provided for in a:

a) SPECI. b) METAR. c) TAF.

FOR THE FOLLOWING QUESTIONS, REFER TO THE METAR BELOW: METAR FABL 180930Z 08015G25KT 220V040 2500 6500NE R17L/1300U +TSSHRA FEW035 BKN070 24/02 Q1011 WS RWY17R BECMG FM 1000Z TL1030Z 14030KT 2000 BR SCT030CB FEW040 8. The reported wx is:

a) Mist. b) Wind shear. c) Heavy thunderstorms with rain showers.


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9. 5 to 7 oktas of cloud were reported at:

a) 7 000 AMSL. b) 700’ AGL. c) 7 000’ AGL.

10. RVR is:

a) Improving. b) Deteriorating. c) Not reported. THE FOLLOWING QUESTION, REFER TO THE TAF BELOW: TAF FAPE 240300Z 240615 26020KT 9999 BKN020 TEMPO 5000 -RA SCT008 BKN0l5 BKN100 TEMPO 0608 SCT060CB BECMG 1214 13020KT. 11. The forecast period is:

a) On the 24th from 03:00 Z. b) On the 24th from 06:00 Z to 15:00 Z. c) On the 24th from 03:00 Z till 15:00 Z the following day.


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Page 209

Annex A

Sample Exams


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Sample Paper 1 1. When the lifting action ceases in unstable conditions the raising air will:

a) cease to rise and stay where it is; b) continue to rise; c) cease to rise and return to its original level. 2. The characteristics of rime ice and conditions most favourable for its formation

are:

a) Opaque, rough appearance, tending to spread back over the aircraft’s surfaces. Most frequently encountered in cumuliform cloud;

b) Milky, granular appearance, forming on leading edges and accumulating forward into the air stream encountered in stratiform clouds and temperatures -10 C to -20 C;

c) Transparent appearance and tending to take the shape of the surface on which it freezes. Encountered in stratiform clouds at temperatures only

slightly below freezing. 3. The rate at which the atmospheric pressure decreases with height is:

a) greater in warm air than in cold air: b) greater in cold air than in warm air; c) the same in cold air and warm air. 4. The characteristics of unstable air are: Visibility Precipitation Clouds a) poor steady stratus; b) good showers cumulus: c) good steady stratus. 5. The characteristics of unstable air are: a) turbulence and good surface visibility; b) turbulence and poor surface visibility;

c) smooth conditions and good surface visibility. 6. The saturated adiabatic lapse rate is less than the dry adiabatic lapse rate

because:

a) its rate of ascent is less; b) latent heat is released during the saturated adiabatic process; c) water vapour does not cool as rapidly as air. 7. The most common cooling process associated with cloud formation is: a) the adiabatic cooling of air; b) cooling by the radiation of heat to the ground and higher layers; c) cooling by mixing with colder air.


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8. During a flight in conditions of haze or mist a pilot will have:

a) a better range of visibility by looking down-sun; b) a better range of visibility by looking up-sun; c) the same range of visibility regardless in which direction he looks

relative to the sun. 9. If the wind blows across a mountain range, mountain waves and turbulence can

always be anticipated on the lee side of the mountain if the:

a) windspeed is less than 20 knots and the air is unstable; b) wind is stronger than 30 knots and the air is unstable; c) wind is stronger than 30 knots and the air is stable. 10. SPECI messages are:

a) routine weather report from an aerodrome; b) special messages giving information regarding the occurrence of expected

occurrence of adverse weather conditions at or near the aerodrome c) special messages giving information regarding the conditions of or changes in

the meteorological forecast service at an aerodrome 11. The air ahead of the warm front is colder than the air behind the overtaking

cold front at:

a) cold front occlusion b) warm front occlusion; c) stationary front. 12. Warm frontal weather is associated with a: a) lowering cloud base, precipitation and reduced visibility; b) lowering cloud base, strong winds and good visibility; c) narrow bands of clouds, thunderstorms and good visibility. 13. On a particular flight in the southern hemisphere the winds at 5000 feet AGL are

north-easterly while the surface winds are easterly The difference in wind direction is primarily due to:

a) a pressure gradient increasing with altitude;

b) a stronger Coriolis force at the surface; c) friction between the wind and the surface. 14. The intensity and extent of the weather at a front depends upon:

a) the moisture content and stability of the warm air mass; b) the moisture content and stability of the cold air mass; c) the temperature of the cold air mass. 15. When crossing a cold front at a high altitude the change in the temperature and

wind direction will be:

a) less than the change at a lower altitude; b) greater than the change at a lower altitude;


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c) the same as the change at a lower altitude. 16. The type of precipitation associated with thick nimbostratus clouds is:

a) rain or snow showers;

b) continuous rain; c) hard or soft hail 17. When crossing a cold front in the southern hemisphere from the warm to the

cold sector, there will be:

a) a backing in the wind direction; b) a veering in the wind direction; c) no change in the wind direction. 18. The standard temperature at 20 000 feet is:

a) 23,4 F; b) -12,3 F; c) -29,2 F. 19. Approaching the equator, the coriolis effect:

a) becomes very small b) becomes larger; c) changes direction. 20. If there is no change in the pressure distribution the surface wind (southern

hemisphere), at mid-day will:

a) increase and back; b) decrease and veer c) decrease and back. 21. Absolute instability exists in the atmosphere when:

a) the ELR is greater than the DALR; b) the ELR is less than the SALR; c) the ELR lies between the DAIR and the SALR 22. Cumulus clouds are always associated with severe icing due to:

a) strong vertical currents producing a predominance of large supercooled water droplets in the clouds;

b) strong vertical currents producing a predominance of small supercooled water droplets in the clouds;

c) the freezing level being at a lower height 23. Thunderstorms usually associated with heavy hail showers and destructive

winds are:

a) warm front thunderstorms; b) squall line thunderstorms; c) thunderstorms in the dissipating stage.


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24. Wind shear occurs:

a) only at high altitudes in the vicinity of jet streams; b) at any level, and it may be associated with change in the wind speed or

direction or both; c) primarily at lower altitudes in the vicinity of mountain waves. 25. Rising air becomes colder because the: a) pressure decreases with height and the air expands; b) surrounding air is colder at higher levels; c) water vapour in the air becomes less at increased heights. 26 A cold breeze blowing down a mountain slope at night is known as:

a) an anabatic wind; b) a katabatic wind; c) a berg wind. 27. Tropical cyclones develop in most tropical oceans:

a) on the equator; b) between 2 degrees and 5 degrees from the equator; c) between 5 degrees and 25 degrees from the equator. 28. The winds that blow from opposite directions in the summer and winter in

certain regions of the tropics are called:

a) a trade winds; b) monsoon winds; c) doldrums. 29. When the reported pressure indicates a more or less continuous fall, the

following type of weather can be anticipated:

a) a rising cloud base, due to the adiabatic warming as a result of subsiding; b) a general drying out of the atmospheric which will result in increased

visibility; c) a general increase, m the cloud cover, increasing windspeed and a

possibility of precipitation. 30. Hoar frost is likely to form on an aircraft when:

a) the aircraft is flying through rain and the temperature is below 0 degrees C: b) it is descending from an altitude where the outside temperature was above

freezing into cold moist air; c) the aircraft is parked outside on a fine clear might and the temperature drops

below 0 degrees C.


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31. TAF FAJS 260615 04008KT 9999 SCT020 BKN080 BECMG 0810 33013KT SCT045

BKN080 BECMG 1013 SCT045CB SCT045 BKN080 TEMPO 1315 5000 TSRA = Rain showers are expected at FAJS :

a) between 0600 and 1500; b) after 1300: c) between 1300 and 1500. 32. The expected WX on the route between FAJS and Alexander Bay (SIG WX2) at FL160 is: a) Patches of cumulus cloud with no icing; b) 5/8 Alto-Cumulus with icing; c) 4/8 to 7/8 Alto-Stratus with icing. 33. The atmospheric pressure reported at MAPUTO (SYNOP 1)is:

a) 1012 hPa:

b) 1108 hPa c) 1008 hPa. 34. According to the information on Significant WX 1, the highest freezing level will be in

the:

a) Cape Town area; b) Windhoek area; c) Durban area. 35. TAF FABL 050615 02010KT 9999 SCT040CB BECMG 0810 35014KT SCT040

BKN100 TEMPO 1215 4000 TSRA SCT030CB = The expected surface wind at FABL at 0900 on the 05th, is:

a) 020/10 Kts;

b) 310/l0Kts; c) 350/l4Kts 36. The direction of the drift on a flight from DURBAN to PORT ELIZABETH at

FL100 (Wind A) will be:

a) to the left; b) to the right: c) none. 37. The tops of the clouds in the vicinity of GABERONE (SIGNIFICANT WX 1) are

expected to be at:

a) 9000 feet AGL; b) 9000 feet AMSL; c) none of the above.


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38. FAPE 110615 14015KT SCT010 OVC020 PROB30 TEMPO 0615 6000 –RA BKN006

BECMG 1113 09020G30KT= During the forecast period, the visibility at PORT ELIZABETH (FAPE) will be:

a) more than 10 km reducing to 6 km at 0600 on the 05th. b) more than 10 km c) more than 10 km with a 30 % possibility of reducing to 6 km at times between 0600 and 1500 on the 05th.

39. TAF FAGG 260609 260609z 11010KT 9999 SCT010 OVC025 PROB30 TEMPO

0609 4000 RA= The term 0609 as used in the TAF for George Airport (FAGG), means:

a) The tops of the clouds will be between 6000 and 9000 feet above means sea

level; b) a temporary change in the visibility will occur between 0600 and 0900;

c) The temperature at 0600 at the airport will be 09 ْC. 40. The WIND DIRECTION AT fact (SYNOP 1) is:

a) 220/25

b) 300/30 c) 150/30 41. The base of the clouds in the Bloemfontein area (SIGNIFICANT WX 2) are:

a) 1200 feet AGL; b) 12000 feet AGL; c) unknown. 42. The type and height of the clouds that will be encountered on a flight from

KEETMANSHOOP to JAN SMUTS (SIGNIFICANT WX 2) will be

a) Cu at 2000 feet becoming 13000 feet in the FAJS area; b) Clear, then becoming Cu with tops 14000ft, and then in the FAJS area broken

Ac. c) Ac and Cu at 14000 feet. 43. TAF FWKI 031430 040024 CAVOK 1001 =

The TAF for FWKI is valid for the period:

a) from 1430 onwards on the 04th; b) 0400 to 1400; c) 0000 to 2400 on the 05th. 44. TAF FAAB 060600 060615 VRB05KT 9999 SCT005 PROB30 TEMPO 0607 1000FG

BKN002 FM08 13012KT CAVOK BECMG 0912 21015KT= The expected landing conditions at FAAB at 0630 will be:

a) visibility 1000 m with fog at 200 feet; b) Visibility 10 km with cloud at 500 feet; c) visibility 1000 with cloud at 500 feet.


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45. TAF FAJS 060615Z 04008KT 9999 SCT020 BKN080 BECMG 0810 33013KT

SCT045 BKN080 BECMG 10131 SCT045CB SCT045 BKN080 TEMPO 1315 5000 TSRA= During the thunderstorm at FAJS, the visibility is expected to drop to:

a) 9900 metres;

b) 10km; c) 5000 metres. 46. The term BKN as used in a TAF means:

a) 8/8 cloud cover

b) 3/8 to 4/8 cloud cover; c) 5/8 to 7/8 cloud cover. 47. The cloud conditions reported at (SHIP B, CHART 1) were:

a) BKN Stratus fractus b) OVC Cumulus

c) OVC Sc 48. The wind direction and speed at DURBAN (CHART 1) is

approximately:

a) 040 degrees / 10 Kts; b) 220 degrees / 05 Kts; c) 220 degrees / 10 Kts. 49. The weather at the time of observation at Diaz Pt (CHART 1) is:

a) No significant weather;

b) Thunderstorms; c) Sky obscured. 50. The temperature and dewpoint temperature at (SHIP C, CHART 1) is:

a) Temp 15°C/DP7°C; b) Temp 28°C/DP7°C; c) Temp 28°C/DP unknown. 51. The QNH at SHIP B (CHART 1) at 1100 Z was: a) 1010.4 hPa, b) 1008.2 hPa c) 1012.6 hPa. 52. Which minimum temperature is most conducive to aircraft icing in stratiform

cloud?

a) -2°C to -15° C; b) 0°C to -15° C; c) 0°C to -10° C.


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Chart 1 / Synop 1


Page 218

Sig Wx 1


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Sig Wx 2


Page 220

Wind A


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Sample Exam Paper 2 1. The rotational flow of air around high and low pressure systems is primarily caused

by: a) Cyclostrophic forces; b) Coriolis force; c) Buys Ballots Law.

2. A Fohn or Chinook wind is associated with one of the following geographical features:

a) A cool ocean; b) Mountain range; c) A valley.

3. A changing wind direction from 320 degrees through to 017 degrees is called:

a) Backing; b) Veering in the Northern Hemisphere and Backing in the Southern Hemisphere; c) Veering.

4. In the Southern Hemisphere the surface wind at an inland aerodrome during the

morning will tend to:

a) Back and increase; b) Veer and increase; c) Increase with no change in direction.

5. If you were flying through a front in the Southern Hemisphere and you wished to

maintain your track, you would:

a) Alter heading to the left; b) Alter heading to the right; c) Any heading change would depend upon the direction you are flying through

the front. 6. Coriolis force in the Southern Hemisphere causes a deflection to:

a) The West; b) The right; c) The left.

7. Surface winds tend to flow across the isobars at an angle rather than parallel to the

isobars. This is due to:

a) Coriolis force; b) Cyclostrophic force; c) Surface friction.


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8. Where would the most frequent formation of radiation fog occur:

a) Anticyclones and ridges; b) Troughs of low pressure; c) Depression and Cols.

9. Under which of the following conditions does advection fog not form:

a) Skies are overcast, or nearly so; b) Wind speeds exceed 8 - 10 knots; c) The wind is calm.

10. Sheets of cloud which produce a halo around the sun are:

a) Altostratus; b) Cirrostratus; c) Stratocumulus.

11. Which of the following winds blow into the ITCZ:

a) Monsoons; b) Westerlies; c) Trade winds.

12. With reference to “Trigger Action”, the type of cloud which is formed is:

a) Stratus; b) Altocumulus; c) Cumuloform type cloud.

13. Moist stable air flowing upslope may be expected to produce:

a) Stratocumulus cloud; b) Stratus-type cloud; c) A temperature inversion

14. Good visibility is normally associated with:

a) Stratus-type cloud; b) A temperature inversion; c) Unstable air.

15. A steep environment lapse rate in a moist air mass would produce;

a) Thunderstorms; b) Stratus cloud with drizzle; c) Low lying fog.

16. Where, in Southern Africa, is advection fog predominant:

a) Natal midlands; b) South West coastal belt; c) Eastern Transvaal escarpment.


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17. When the visibility is less than 1000 metres, the weather phenomena is known as:

a) Mist; b) Haze; c) Fog.

18. Forecast winds: 7000ft AGL - 270/20 surface wind - 315/5 The difference in direction and speed is primarily due to:

a) The stronger coriolis force at the surface; b) Stronger pressure gradient at higher altitudes; c) The effect of surface friction.

19. The presence of ice pellets is evidence that:

a) There are thunderstorms present; b) There is freezing rain at higher altitudes; c) A cold front has passed.

20. At what height AGL would you expect the base of cumuloform cloud to form if the

surface temperature is 24 degrees centigrade and the dew point is 12 degrees centigrade:

a) 4000 ft; b) 4900 ft; c) 8800 ft.

21. The characteristics of unstable air are:

a) Turbulence and good surface visibility; b) Nimbostratus cloud and good visibility; c) Nimbostratus cloud and rain.

22. Which of the following features is associated with temperature inversions:

a) Unstable air; b) Stable air; c) Thunderstorms.

23. Saturated air that is forced to rise up a mountain will cool at the rate of:

a) 1.98 Cº/1000 ft; b) 3 Cº/1000ft; c) 1.5 Cº/1000 ft.

24. Lenticular altocumulus standing clouds are likely to indicate:

a) Jet streams; b) Heaving icing conditions; c) Strong turbulence.


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25. A cold air mass moving over a warm land mass will:

a) Become stable; b) Become unstable; c) Will cause poor surface visibility.

26. Which of the following weather phenomena is always associated with the passage of

a frontal system:

a) An abrupt temperature decrease; b) A wind change; c) An abrupt pressure decrease.

27. Which groups of clouds are least likely to produce or contribute to aircraft icing:

a) High cloud; b) Middle cloud; c) Clouds of vertical development.

28. In which environment is aircraft icing most likely to have the highest rate of

accumulation:

a) Heavy wet snow; b) Cumulonimbus cloud; c) Freezing rain.

29. A layer of turbulent cloud formed by strong wind is comprised of the following: Cloud tops - 7800 ft AMSL Base - 7000 AMSL Ground elevation - 5000 ft surface Surface temp +7º C

Icing can be expected: a) At altitude 7670 ft and above; b) Between 7670 ft and 7800 ft; c) Between 7000 and 7800 ft.

30. During the lifecycle of a thunderstorm, which stage is characterised predominantly by

downdraughts:

a) The mature stage when precipitation commences; b) The dissipating stage with the formation of the anvil; c) The cumulus stage.

31. The atmosphere is composed of the following gases:

a) Carbon dioxide 21%. Oxygen 78%, Water Vapour 1%. b) Oxygen 21%, Nitrogen 78%, Water Vapour 1% c) Ozone 1%, Carbon Dioxide 78%, Oxygen 21%.


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32. The layer that corresponds to that of ISA with a constant temperature of -56.5 C is

found in:

a) The Stratosphere. b) The Troposphere. c) The Stratopause.

33. An aircraft is at FL220 with the altimeter set. The pilot omits to reset the altimeter for

landing. The destination airfield has an elevation of 580m, with a QNH of 1026.1 hPa. After landing, the altimeter reads:

a) 2 285’ b) 1 517’ c) 1 900’

34. During an altimeter serviceability check, the following indications were observed.

Airfield elevation 5 327’, apron elevation 5 306’. The altimeter with the QFE set reads 80’. The instrument error is:

a) 101’ b) 59’ c) 0’

35. The amount of water vapour in a mass of air expressed as a percentage of the

total amount of water vapour that the mass of air could contain if it was saturated at the same temperature and pressure, is known as the:

a) absolute humidity; b) relative humidity; c) specific humidity.

36. Rising air becomes colder because the

a) pressure decreases with height and the air expands; b) surrounding air is colder at higher levels; c) water vapour in the air becomes less.

37. You are flying at an indicated height of 2000 feet from a high pressure to a low

pressure system. If you maintain the indicated height your true height will:

a) increase b) stay the same c) decrease.

38. The earth’s weather changes are primarily due to one of the following:

a) variation of solar energy received at the surface of the earth; b) movement of the airmass; c) pressure variations over the earth’s surface.


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39. The amount of water vapour which may be held in suspension in the atmosphere

depends largely on the following:

a) The air temperature. b) The stability of the air. c) The dew point temperature.

40. The temperature at an airfield at 3 000’ AMSL is 84°F. This represents:

a) ISA -20C b) ISA-15C c) ISA+17C


a) 32C=0F b) C= 5/9(F-32) c) F=5/9(C-32)

42. A Stevenson screen is used in the measurement of:

a) Wind velocity. b) Temperature c) Rainfall.

43. Landing at Johannesburg International, (elevation 5500’), you are given a QFE

of 840 hPa. The QNH is:

a) 1023 hPa. b) 1017 hPa. c) 1003 hPa.

44. An increase in relative humidity with no change in temperature or pressure will cause:

a) No change in air density. b) A slight decrease in density altitude. c) A slight increase in density altitude.

45. An aircraft flying at a constant flight level from an area of high temperature to an area

of low temperature will experience:

a) An increase in density altitude. b) A decrease in density altitude. c) An increase in density altitude unless there is a substantial increase in relative

humidity.

46. Tropical cyclones develop in most tropical oceans:

a) On the equator. b) Between 5° and 15° from the equator. c) Between 5° and 25° from the equator.


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47. Tropical cyclones would be expected to develop over the southern Indian Ocean:

a) July - November b) June - November c) December - April


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Sample Exam Paper 3 1. During the South African winter the ITCZ:

a) moves well south of the equator; b) moves well north of the equator; c) remains within 5 degrees of the equator.

2. The South Westerly Buster is found on the;

a) South Western coast of South Africa; b) Eastern coast of Southern Africa; c) Cape coast, resulting in the famous “table cloth” cloud over Table

Mountain.

3. A change in visibility, low cloud, surface wind or weather is provided for in a:

a) TAF; b) METAR; c) SPECI.

4. In a TAF printout, if a change is expected to take place lasting less than 1 hour

of duration, then the word used is:

a) TEMPO; b) INTER; c) BECMG.

5. PROB is used in a TAF to indicate:

a) that certain weather phenomena will take place; b) the probability of a change expressed as a percentage; c) the time of a certain change in weather.

THE FOLLOWING QUESTIONS APPLY TO THIS TAF: TAF FVBU 0515 06008 KT CAVOK BECMG 1113 SCT 050 CB SCT 100 PROB 40 TEMPO 1215 8000 - TSRA

6. What is the forecast time period:

a) 0515 LMT to 0600 LMT; b) 0500 UTC to 1500 UTC; c) 0515 UTC to 0600 UTC.

7. What is expected to develop after 1100 HRS:

a) wind direction change to 050 degrees; b) Cumulo-Nimbus cloud at 5000 feet; c) CAVOK.


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8. What is the base of the middle cloud:

a) 1000 feet; b) Unknown; c) 10 000 feet.

9. What will the visibility be in the thunderstorm:

a) 8000 feet; b) 8000 metres; c) more that 10 km.

10. What is the temporary change that will take place at 1200 HRS:

a) a wind change; b) Icing will occur; c) Visibility will drop because of a thunderstorm.

11. The word ‘CAVOK’ signifies:

a) Visibility more than 9000 metres, no low cloud, wind calm, no

precipitation; b) Visibility greater than 10 km, no cloud below 5000 ft, no precipitation

and no thunderstorms; c) Visibility 10 km or more with no low or middle cloud, or fog.

THE FOLLOWING QUESTIONS REFER TO Station level decode, Station No.1

12. The pressure is:

a) 1019 hPa; b) 1016 hPa; c) 1017 hPa.

13. The base of cloud is:

a) 19 000 feet; b) 900 feet; c) 9 000 feet and above.

14. The cloud cover is:

a) 4 octas; b) 6 octas; c) 8 octas.

15. What was the QNH 3 HRS previous:

a) 1016.0 hPa; b) 1015.3 hPa; c) 101 d) 7.4 hPa.


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16. Fog is reported position A, on Synop chart 1. The type of fog is:

a) radiation fog; b) upslope fog; c) advection fog.

THE FOLLOWING QUESTIONS REFER TO SYNOPTIC CHART 1, & THE STATIONLEVEL DECODE PAGE: 17. Why is the temperature 29 C at MAPUTO:

a) high pressure resulting in calm, warm conditions; b) clear sky and in a Col; c) berg winds.

18. What is the temperature and dew point at WINDHOEK – Station No. 2:

a) 17 C and 18 C; b) 21 C and 18 C; c) 21 C and 17 C.

19. Thunderstorms are present in the WINDHOEK area – Station No. 4 (to the

north of Windhoek). The cloud base is:

a) 3000 ft to 5000 ft above mean sea level; b) 3000 ft to 5000 ft above station elevation; c) 3000 to 5000 metres above ground level.

20. What is the reason for the thunderstorms in this area:

a) surface heating; b) trough of low pressure extending south of Angola; c) high pressure to the east, advecting moist air from the north.

21. An aircraft is expected to arrive in CAPE TOWN approximately 3 hours after

the time of observation. The pilot could expect:

a) the visibility to be improving; b) the weather to remain much the same, with the possibility of

deteriorating weather conditions; c) thunderstorms with lowering cloud base.

22. Why is the atmospheric pressure at DURBAN increasing:

a) there is a cold front approaching; b) there is a coastal low moving up the coast to the north; c) there is a high pressure area in the interior moving eastward.

23. The atmospheric pressure at Station No. 3 north east of CAPE TOWN is:

a) 1013.9 hPa; b) 1016.1 hPa; c) 1011.3 hPa.


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24. An aircraft’s track is 355 (T), wind 090/20 kts, flight level 90. The altimeter

would:

a) start to over-read; b) start to under-read; c) remain constant.

25. With reference to SIG WX 1. Flying from JS to DN, the type of cloud that

can be expected:

a) broken scattered cloud; b) isolated Cb in the FAJS area; c) broken strato-cumulus.

26. With reference to the SIG WX 1. The lowest freezing level is in the:

a) FAPE area; b) FADN area; c) FAJS area.

27. With reference to SIG WX 1. The freezing level in the FACT area is:

a) 14 000 feet AGL; b) 14 000 feet AMSL; c) 1000 feet AMSL.

28. With reference to SIG WX 1. The base of the clouds in the FAGG area are:

a) 2000 feet AGL; b) 2000 feet AMSL; c) 5000 feet AMSL.

29. With reference to SIG WX 1. The tops of the clouds in the FAJS area are:

a) 10000 feet AMSL b) 10000 metres AMSL c) 1000 feet AMSL

30. With reference to SIG WX 1. The visibility in the FACT area is:

a) 1 km; b) 1000 feet; c) 9999m or more

31. With reference to WIND A. The wind at FL 150 on the Natal North coast is:

a) 263 degrees 20 kts; b) 43 degrees 7 kts; c) 37 degrees 7 kts.


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32. With reference to WIND A, the temperature at FL 100 (Natal North Coast) is:

a) minus 8º C; b) plus 7º C; c) plus 8º C.

33. What is meant by the term ‘embedded thunderstorm’:

a) thunderstorms are obscured by massive cloud layers; b) thunderstorms are predicted to form in stable air; c) severe thunderstorms lie within a squall line.

34. The structure and formation of different cloud types which form as a result of air

that is forced to rise, depends on:

a) the stability of the air; b) the method by which air is forced to rise; c) the amount of humidity present after lifting occurs.

35. Rime ice could be expected in flight when:

a) in towering cumuliform cloud; b) in small supercooled water droplets with the aircraft surface

temperature 0 C or below; c) in rain at temperatures below 0 C.

36. The Doldrums are:

a) another name for sub-tropical anticyclones; b) weak low pressure areas behind fronts; c) Cols between weak fronts encountered in low latitudes.

37. Buys ballot Law states that: “If you stand with your back to the wind in the southern

hemisphere, the low pressure area will be”:

a) on your left; b) in front of you; c) on your right.

38 An ELR value which falls between those of the SALR and the DALR indicates:

a) absolute stability; b) absolute instability; c) conditional instability.

39. The most severe form of icing is:

a) rime ice; b) rain ice; c) hoar frost.


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40. If 01510 appeared in a TAF printout it would indicate:

a) surface visibility 1510 metres; b) valid from 1500 to 1000 UTC; c) wind 015 / 10 kts TRUE.


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10

21

18

6

163

9


235

Synop Chart 1

A


236

Wind A


237

Sig Wx 1


238


239

Annex B

Answers


240

Chapter 1

1 B 10 B 2 A 11 A 3 B 12 C 4 A 13 B 5 C 14 B 6 A 15 C 7 A 16 C 8 A 17 A 9 B 18 B

Chapter 2

1 A 11 B 2 B 12 A 3 A 13 B 4 B 14 A 5 C 15 A 6 B 16 C 7 B 17 B 8 A 18 A 9 A 19 A

10 C 20 B

Chapter 3

1 B 2 A 3 C 4 A 5 B 6 B 7 A 8 B

Chapter 4

1 B 2 A

Chapter 5

1 B 2 C 3 B 4 A 5 A


241

6 C


242

Chapter 6

1 A 7 A 2 C 8 C 3 B 9 C 4 A 10 A 5 A 11 B 6 A

Chapter 8

1 C 11 B 2 B 12 A 3 A 13 B 4 A 5 A 6 A 7 B 8 C 9 B

10 C Chapter 9

1 B 2 A 3 C 4 A 5 C 6 C 7 C

Chapter 10

1 A 2 A 3 A 4 A 5 B

Chapter 11

1 C 2 B 3 C 4 B 5 C 6 C


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Chapter 13 Synoptics

1 A 2 C 3 B 4 B 5 A 6 B 7 B 8 C 9 B

10 A 11 B

TAF/Metar

1 B 8 C 2 C 9 C 3 B 10 A 4 C 11 B 5 B 6 A 7 A


244

Sample Exam Paper 1

1 B 29 C 2 B 30 C 3 B 31 C 4 B 32 B 5 A 33 A 6 B 34 B 7 A 35 C 8 A 36 A 9 C 37 C

10 B 38 C 11 B 39 B 12 A 40 C 13 C 41 C 14 A 42 B 15 A 43 C 16 A 44 A 17 A 45 C 18 B 46 C 19 A 47 C 20 A 48 B 21 A 49 C 22 A 50 C 23 B 51 A 24 B 52 B 25 A 26 B 27 C 28 B


245

Sample Exam Paper 2

1 B 25 B 2 B 26 B 3 C 27 A 4 A 28 C 5 A 29 B 6 C 30 B 7 C 31 B 8 A 32 A 9 C 33 B

10 B 34 A 11 C 35 B 12 C 36 A 13 B 37 C 14 C 38 A 15 A 39 A 16 B 40 A 17 C 41 B 18 C 42 B 19 B 43 A 20 B 44 C 21 A 45 B 22 B 46 B 23 C 47 C 24 C


246

Sample Exam Paper 3

1 B 21 B 2 B 22 C 3 C 23 C 4 A 24 A 5 B 25 C 6 B 26 A 7 B 27 B 8 C 28 B 9 B 29 A

10 C 30 C 11 B 31 A 12 B 32 B 13 C 33 A 14 C 34 A 15 A 35 B 16 C 36 A 17 C 37 C 18 B 38 C 19 B 39 B 20 B 40 C

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Corrected_ATPL_Meteorology[1].pdf